View Compilation Error

0_0_36516346_14604.cpp: In function 'long long int mex(long long int)':
0_0_36516346_14604.cpp:8:8: error: 'b' was not declared in this scope
  if (n<b) return sg[n]=0;
        ^
0_0_36516346_14604.cpp: In function 'int main()':
0_0_36516346_14604.cpp:28:3: error: 'a' was not declared in this scope
   a-=b;
   ^
0_0_36516346_14604.cpp:28:6: error: 'b' was not declared in this scope
   a-=b;
      ^