#include <iostream>
using namespace std;
void main()
{
    int n;
    int a[998];
    cin>>n;
    int i;
    int s=0;
    int smax=0;
    int imid=0;
    for(i=0;i<n;i++)
    {
        cin>>a[i];
    }
    for(i=0;i<n-2;i++)
    {
        s=a[i]+a[i+1]+a[i+2];
        if(s>smax)
        {
            smax=s;
            imid=i+2;
        }
    }
    cout<<smax<<" ";
    cout<<imid;
}

What did i do wrong ?

why have you taken array of size 998? make it 1000.