#include <bits/stdc++.h>

using ll = long long;

using namespace std;

int main()
{
    int n;
    scanf("%d", &n);

    vector<int> v(n);
    for (int i = 0; i < n; ++i)
        scanf("%d", &v[i]);

    vector<int> v2(v),v3(n);
    sort(v2.begin(), v2.end());
    reverse_copy(v2.begin(),v2.end(),v3.begin());

    if (v == v2 || v == v3)
    {
        printf("-1\n");
        return 0;
    }

    constexpr int maxa = 300000001;

    vector<int> lmin(n + 1), rmin(n + 1);
    lmin[0] = maxa;
    rmin[n] = maxa;

    for (int i = 0; i < n; ++i)
    {
        lmin[i + 1] = min(lmin[i], v[i]);

        rmin[n - i - 1] = min(rmin[n - i], v[n - i - 1]);
    }

    int mint{ maxa };
    for (int i = 1; i < n; ++i)
    {
        if (v[i] > lmin[i] && v[i] > rmin[i + 1])
            mint = min(mint, lmin[i] + v[i] + rmin[i + 1]);
        if (v[i] < lmin[i] && v[i] < rmin[i + 1])
            mint = min(mint, lmin[i] + v[i] + rmin[i + 1]);
    }

    if (mint != maxa)
        printf("%d\n", mint);
    else
        printf("-1\n");

    return 0;
}