diff --git a/klotski/common.cc b/klotski/common.cc
index 21a2118..8118f23 100644
--- a/klotski/common.cc
+++ b/klotski/common.cc
@@ -7,7 +7,7 @@ uint32_t Common::range_reverse(uint32_t bin) { // reverse binary every 2-bits
     return ((bin << 2) & 0xCCCCCCCC) | ((bin >> 2) & 0x33333333);
 }
 
-bool Common::check_case(uint32_t head, uint32_t range) { // check whether the case is valid
+bool Common::check_case(uint32_t head, uint32_t range) { // whether the head and range is valid
     uint32_t mask = 0b110011 << head; // fill 2x2 block
     for (int addr = 0; range; range >>= 2) { // traverse every 2-bits
         while (mask >> addr & 0b1) {
@@ -34,7 +34,39 @@ bool Common::check_case(uint32_t head, uint32_t range) { // check whether the ca
                 mask |= 0b11 << addr; // fill 1x2 block
                 break;
         }
-        // TODO: should we ensure that low-bits as 0?
     }
     return true; // valid case
 }
+
+bool Common::check_case_safe(uint32_t head, uint32_t range) { // whether the case is valid
+    uint32_t space_num = 0;
+    uint32_t mask = 0b110011 << head; // fill 2x2 block
+    for (int addr = 0; range; range >>= 2) { // traverse every 2-bits
+        while (mask >> addr & 0b1) {
+            ++addr; // search next not filled block
+        }
+        switch (range & 0b11) {
+            case 0b00: // space block
+                ++space_num;
+            case 0b11: // 1x1 block
+                if (addr > 19) { // invalid address
+                    return false;
+                }
+                mask |= 0b1 << addr; // fill 1x1 block
+                break;
+            case 0b10: // 2x1 block
+                if (addr > 15 || mask >> (addr + 4) & 0b1) { // invalid address
+                    return false;
+                }
+                mask |= 0b10001 << addr; // fill 2x1 block
+                break;
+            case 0b01: // 1x2 block
+                if (addr > 18 || (addr & 0b11) == 0b11 || mask >> (addr + 1) & 0b1) { // invalid address
+                    return false;
+                }
+                mask |= 0b11 << addr; // fill 1x2 block
+                break;
+        }
+    }
+    return space_num >= 2; // at least 2 space
+}
diff --git a/klotski/common.h b/klotski/common.h
index 8c80a6c..505bef1 100644
--- a/klotski/common.h
+++ b/klotski/common.h
@@ -6,4 +6,5 @@ class Common {
 public:
     static uint32_t range_reverse(uint32_t bin);
     static bool check_case(uint32_t head, uint32_t range);
+    static bool check_case_safe(uint32_t head, uint32_t range);
 };
diff --git a/klotski/common_code.cc b/klotski/common_code.cc
index fae0267..e4e403d 100644
--- a/klotski/common_code.cc
+++ b/klotski/common_code.cc
@@ -7,5 +7,5 @@ bool CommonCode::check(uint64_t common_code) {
     if (head >= 16 || (head & 0b11) == 0b11) { // check 2x2 block address
         return false; // invalid common code
     }
-    return Common::check_case(head, Common::range_reverse(range)); // check by head and range
+    return Common::check_case_safe(head, Common::range_reverse(range)); // check by head and range
 }
diff --git a/klotski/main.cc b/klotski/main.cc
index 47799b7..4b1f2dd 100644
--- a/klotski/main.cc
+++ b/klotski/main.cc
@@ -46,22 +46,28 @@ int main() {
 //    }
 
 //    uint32_t sum = 0;
-    for (uint32_t head = 0; head < 1; ++head) {
-        auto prefix = (uint64_t)head << 32;
-        for (uint64_t range = 0; range < 0x100000000; ++range) {
-            uint64_t code = prefix | range;
-            if (CommonCode::check(code)) {
-                printf("%09lX\n", code);
+//    for (uint32_t head = 0; head < 1; ++head) {
+//        auto prefix = (uint64_t)head << 32;
+//        for (uint64_t range = 0; range < 0x100000000; ++range) {
+//            uint64_t code = prefix | range;
+//            if (CommonCode::check(code)) {
+//                printf("%09lX\n", code);
 //                ++sum;
-            }
-
+//            }
 //            if (range % 0x1000000 == 0) {
 //                std::cout << range / 0x1000000 << std::endl;
 //            }
+//        }
+//    }
+//    std::cout << "sum: " << sum << std::endl;
+
+    uint64_t error_code = 0x02EFFF7C0;
 
-        }
+    if (CommonCode::check(error_code)) {
+        std::cout << "true" << std::endl;
+    } else {
+        std::cout << "false" << std::endl;
     }
-//    std::cout << "sum: " << sum << std::endl;
 
     return 0;
 }