ural 1047 Simple Calculations

]#include<iostream>
using namespace std;
double sum,c,temp;
double aN,a0;
int n;
int main()
{
int i,j,k;
scanf("%d",&n);
scanf("%lf%lf",&a0,&aN);
for(i=0;i<n;i++)
{
scanf("%lf",&temp);
c+=temp;
sum+=c;
}
sum*=2;
printf("%.2lf/n",(double)(aN+n*a0-sum)/(n+1));
return 0;
}