#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define elif else if
#define sp(x) fixed << setprecision(x)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define sz(x) (int)x.size()
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
const ld EPS = 1e-10;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

int main(){
    int N;
    cin >> N;
    int A[N];
    rep(i, N) cin >> A[i];
    int ans1[N], ans2[N];
    rep(i, N) ans1[i] = ans2[i] = A[i];
    set<int> l, r;
    set<int>::iterator it;
    rep(i, N){
        it = l.lower_bound(A[i]);
        if(it == l.end()) ans1[i] = inf;
        else ans1[i] += *it;
        if(sz(l) == 0) ans2[i] = inf;
        else{
            int u = *l.begin();
            if(u > A[i]) ans2[i] = inf;
            else ans2[i] += u;
        }
        l.insert(A[i]);
    }
    rep3(i, N-1, 0){
        it = r.lower_bound(A[i]);
        if(it == r.end()) ans1[i] = inf;
        else ans1[i] += *it;
        if(sz(r) == 0) ans2[i] = inf;
        else{
            int u = *r.begin();
            if(u > A[i]) ans2[i] = inf;
            else ans2[i] += u;
        }
        r.insert(A[i]);
    }
    int res = inf;
    rep(i, N){
        chmin(res, min(ans1[i], ans2[i]));
    }
    if(res == inf) cout << -1 << endl;
    else cout << res << endl;
}