View Compilation Error

0_0_36491231_28223.cpp: In function 'int main()':
0_0_36491231_28223.cpp:15:45: error: 'abs' was not declared in this scope
             sum+=abs(x[i-1]*y[i]-x[i]*y[i-1]);
                                             ^
0_0_36491231_28223.cpp:17:37: error: 'abs' was not declared in this scope
         sum+=abs(x[n]*y[1]-y[n]*x[1]);//首尾相连
                                     ^