View Compilation Error

0_0_27980630_22291.cpp: In function 'int main()':
0_0_27980630_22291.cpp:30:21: error: 'a' was not declared in this scope
   printf("%I64d\n",(a-b)%mod);
                     ^
0_0_27980630_22291.cpp:30:23: error: 'b' was not declared in this scope
   printf("%I64d\n",(a-b)%mod);
                       ^