poj 2601 Simple calculations

Simple calculations

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 5803		Accepted: 2947

Description

There is a sequence of n+2 elements a₀, a₁, ..., a_n+1 (n <= 3000, -1000 <= a_i <=1000). It is known that a_i = (a_i-1 + a_i+1)/2 - c_i for each i=1, 2, ..., n.
You are given a₀, a_n+1, c₁, ... , c_n. Write a program which calculates a₁.

Input

The first line of an input contains an integer n. The next two lines consist of numbers a₀ and a_n+1 each having two digits after decimal point, and the next n lines contain numbers c_i (also with two digits after decimal point), one number per line.

Output

The output file should contain a₁ in the same format as a₀ and a_n+1.

Sample Input

1
50.50
25.50
10.15

Sample Output

27.85

#include <iostream>
using namespace std;
int main()
{    
    int n,i;    
    double a,b,temp;    
    double sum=0.0;
    scanf("%d",&n);    
    scanf("%lf%lf",&a,&b);    
    for (i=0;i<n;i++)    
    {        
        scanf("%lf",&temp);        
        sum+=(n-i)*temp;    
    }    
    sum=-sum*2;    
    sum+=n*a+b;    
    sum/=n+1;    
    printf("%.2lf\n",sum);    
    return 0;
}