View Compilation Error

0_0_21648317_17438.cpp: In function 'int main()':
0_0_21648317_17438.cpp:76:13: error: 'b' was not declared in this scope
             b- %= mod; bx %= mod; by %= mod;
             ^
0_0_21648317_17438.cpp:76:16: error: expected primary-expression before '%=' token
             b- %= mod; bx %= mod; by %= mod;
                ^