View Compilation Error

0_0_37864989_8437.cpp: In function 'int main()':
0_0_37864989_8437.cpp:34:20: error: 'gcd' was not declared in this scope
         if(gcd(a, b) == 1) {
                    ^
0_0_37864989_8437.cpp:38:29: error: 'gcd' was not declared in this scope
         int flag = (gcd(a, b) == 2);
                             ^