【杭电acm】2086 A1 = ？

 1 #include <stdio.h>
 2 
 3 int main() {
 4     double a0, an1, c;
 5     double a1;
 6     int n, i;
 7 
 8     while (scanf("%d", &n)!=EOF && n) {
 9         scanf("%lf %lf", &a0, &an1);
10         a1 = an1+n*a0;
11         for (i=0; i<n; ++i) {
12             scanf("%lf", &c);
13             a1 -= 2*c*(n-i);
14         }
15         a1 /= (n+1);
16         printf("%.2lf\n", a1);
17     }
18 
19     return 0;
20 }