View Compilation Error

0_0_34016701_23082.cpp: In function 'int main()':
0_0_34016701_23082.cpp:91:17: error: 'n' was not declared in this scope
     scanf("%d",&n);
                 ^
0_0_34016701_23082.cpp:93:53: error: 'a' was not declared in this scope
         for(int i = 1; i <= n; ++i) scanf("%lf%lf",&a[i].x,&a[i].y);
                                                     ^
0_0_34016701_23082.cpp:94:14: error: 'a' was not declared in this scope
         sort(a + 1,a + 1 + n,compare_x);
              ^