View Compilation Error

0_0_15832744_7900.cpp:4:1: error: '_int64' does not name a type
 _int64 fk(int x)
 ^
0_0_15832744_7900.cpp: In function 'int main()':
0_0_15832744_7900.cpp:18:23: error: 'fk' was not declared in this scope
  printf("%I64d\n",fk(n));
                       ^