#include<bits/stdc++.h>
using namespace std;
int n,a[10005],s[10005],sum;
int main(){
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        cin>>a[i];
        s[i]=s[i-1]+a[i];
    }
    for(int i=1;i<=n;i+=2)
        for(int j=1;j<=___;j++)
            sum+=s[____]-s[__];
    cout<<sum;
    return 0;
}