#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);

    int N, ans=2e9, x, y;
    cin >> N;

    vector<int> A(N+1);
    for(int i=1; i<=N; ++i) cin >> A[i];

    set<int> sl, sr;
    for (int i=1; i<=N; ++i) sr.insert(i);

    for (int i=1; i<=N; i++){
        sr.erase(A[i]);
        if (sl.size() == 0 || sr.size() == 0){
            sl.insert(A[i]);
            continue;
        }
        x = *sl.begin();
        y = *sr.begin();
        if (x < A[i] && y < A[i]) ans = min(ans, x+y+A[i]);
        x = *sl.rbegin();
        y = *sr.rbegin();
        if (x > A[i] && y > A[i]) ans = min(ans, x+y+A[i]);
        sl.insert(A[i]);
    }

    if (ans == 2e9) cout << -1;
    else cout << ans;
    cout << '\n';

    return 0;
}