#include<bits/stdc++.h>

using namespace std;
using ll = long long;
const int INF = 1e9;


int main()
{
    int n; cin >> n;
    vector<int> a(n), lef(n), rig(n);
    for(int i = 0; i < n; i++)cin >> a[i];
    lef[0] = a[0], rig[n - 1] = a[n - 1];

    for(int i = 1; i < n; i++)lef[i] = min(lef[i - 1], a[i]);
    for(int i = n - 2; i >= 0; i--)rig[i] = min(rig[i + 1], a[i]);


    int nin = INF;

    for(int i = 1; i < n - 1; i++) {
        int l = lef[i - 1], r = rig[i + 1];
        if((l < a[i] && a[i] > r) || (l > a[i] && a[i] < r)) {
            nin = min(nin, l + a[i] + r);
        }
    }
    cout << (nin == INF ? - 1: nin) << endl;
}