#pragma once

//https://atcoder.jp/contests/dp/tasks/dp_t

#include <bits/stdc++.h>

#define M_PI       3.14159265358979323846   // pi

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<ll> VI;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;

#define rep(a,n) for(ll a = 0;a < n;a++)
#define repi(a,b,n) for(ll a = b;a < n;a++)

#include <bits/stdc++.h>
using namespace std;

static const ll INF = 1e15;

static const ll mod = 1e9 + 7;

ll solve(const vector<ll>& as) {
	ll res = -INF;
	ll s = 0;
	rep(j, as.size()) {
		s = max(s + as[j], as[j]);
		res = max(res, s);
	}
	return res;
}


int main() {
	int n;
	cin >> n;
	vector<ll> vs(n);
	rep(i, n) cin >> vs[i];
	set<ll> right;
	rep(i, n) right.insert(vs[i]);
	set<ll> left;
	left.insert(vs[0]);
	right.erase(vs[0]);
	ll lower = INF;
	for (int i = 1; i < n-1; i++) {
		ll c = vs[i];
		right.erase(c);
		auto l = left.lower_bound(vs[i]);
		auto r = right.lower_bound(vs[i]);
		if (l != left.end() && r != right.end()) {
			lower = min(lower, *l + *r + c);
		}
		if (*left.begin() < c && *right.begin() < c) {
			lower = min(lower, *left.begin() + *right.begin() + c);
		}
		left.insert(c);
	}
	if (lower != INF) {
		cout << lower << endl;
	}
	else {
		cout << -1 << endl;
	}
	return 0;
}