View Compilation Error

0_0_25386041_21726.cpp: In function 'int main()':
0_0_25386041_21726.cpp:18:18: error: 'a' was not declared in this scope
             cin>>a[i].x>>a[i].y;
                  ^
0_0_25386041_21726.cpp:21:14: error: 'a' was not declared in this scope
         sort(a+1,a+1+2*n,cmp);
              ^