View Compilation Error

0_0_35419977_13856.cpp: In function 'int main()':
0_0_35419977_13856.cpp:16:57: error: 'abs' was not declared in this scope
             sum += abs(x[i - 1] * y[i] - x[i] * y[i - 1]);
                                                         ^
0_0_35419977_13856.cpp:18:45: error: 'abs' was not declared in this scope
         sum += abs(x[n] * y[1] - y[n] * x[1]); //首尾相连
                                             ^