This is my code.
#include <iostream>
#include <vector>
using namespace std;
int main()
{
    int n,a,k=0;
    cin>>n;
    vector <int> v;
    vector <int> sum;
    vector <int> index;
    for(int i=0; i<n; i++)
    {
        cin>>a;
        v.push_back(a);
    }
    for(int i=1; i<n-1; i++)
    {
        sum.push_back(v[i-1]+v[i]+v[i+1]);
        index.push_back(i);
    }
    int max=sum[0];
    for(int i=1; i<n; i++)
    {
        if(sum[i]>max) {max=sum[i]; k=i;}
    }
    cout<<max<<" "<<index[k]+1;
    system("pause");
    return 0;
}

If you look carefully, you don't get n sums and indexes, but actually n-2 sums and indexes.