View Compilation Error

0_0_2174650_1365.cpp: In function `void ca()':
0_0_2174650_1365.cpp:38: error: `abs' undeclared (first use this function)
0_0_2174650_1365.cpp:38: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_2174650_1365.cpp: In function `void back(int, int, int)':
0_0_2174650_1365.cpp:69: error: `abs' undeclared (first use this function)