View Compilation Error

0_0_15900122_15629.cpp:4:1: error: '_int64' does not name a type
 _int64 gb(int m,int n)
 ^
0_0_15900122_15629.cpp: In function 'int main()':
0_0_15900122_15629.cpp:37:21: error: 'gb' was not declared in this scope
             a=gb(a,b);
                     ^