View Compilation Error

0_0_4307267_15999.cpp:4: error: `main' must return `int'
0_0_4307267_15999.cpp: In function `int main(...)':
0_0_4307267_15999.cpp:16: error: `abs' undeclared (first use this function)
0_0_4307267_15999.cpp:16: error: (Each undeclared identifier is reported only once for each function it appears in.)