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

int main()
{
	int N;
	cin >> N;
	vector<long long> A( N );
	for( int i = 0; i < N; i++ ) {
		cin >> A[i];
	}

	const long long INF = INT64_MAX / 2;
	vector<long long> mib( N );
	long long mi = INF;
	for( int i = 0; i < N; i++ ) {
		mi = min( mi, A[i] );
		mib[i] = mi;
	}
	vector<long long> mia( N );
	mi = INF;
	for( int i = N - 1; i >= 0; i-- ) {
		mi = min( mi, A[i] );
		mia[i] = mi;
	}
	long long ans = INF;
	for( int i = 1; i < N - 1; i++ ) {
		long long n = A[i];
		if( mib[i - 1] > n && mia[i + 1] > n ) {
			n += mib[i - 1] + mia[i + 1];
		}
		else if( mib[i - 1] < n && mia[i + 1] < n ) {
			n += mib[i - 1] + mia[i + 1];
		}
		else continue;
		ans = min( ans, n );
	}
	if( ans == INF ) ans = -1;

	cout << ans << endl;
}