View Compilation Error

0_0_12857154_1324.cpp: In function `int main()':
0_0_12857154_1324.cpp:17: error: `f' undeclared (first use this function)
0_0_12857154_1324.cpp:17: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_12857154_1324.cpp: In function `int f(int, int)':
0_0_12857154_1324.cpp:24: error: `int f(int, int)' used prior to declaration