Wrong answer    1 test

#include <iostream>
using namespace std;

int main()
{
    int n, i, x, max;
    int mas[1000];

    cin >> n;
    max = 0;
    for (i = 0; i < n; i++)
    {
        cin >> mas[i];
    }
    max = mas[0] + mas[1] + mas[2];
    x = 1;
    for (i = 1; i < n; i++)
    {
        if (mas[i] + mas[i + 1] + mas[i + 2] > max)
        {
            max = mas[i] + mas[i + 1] + mas[i + 2];
            x = i + 1;
        }
    }
    cout << max << ' ' << x + 1 << endl;
    system("pause");
    return (0);
}

Because for i = n - 1, mas[i + 1] and mas[i + 2] go out of bounds

put the variables above