杭电2086 A=？

1.  # include <iostream>
2. # include <cstdio>
3. using namespace std;
4. int main(){
6. int n,i,j;
7. double  a[2],b[50000],k;
9. while(cin>>n){
10. cin>>a[0]>>a[1];
11. for(i=0;i<n;i++){
12. cin>>b[i];
13. }
14.  
15. double sum = 0;
16. sum = 1.0*n/(n+1)*a[0] + 1.0/(n+1)*a[1];
18. k = 1.0*n*2;
19. for(j=0;j<n;j++){
20. sum -= k/(n+1)*b[j];
21. k-=2.0;
22. }
24. printf("%.2lf\n",sum);
25. }
27. return 0;
28. }