#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>
#include<utility>
#include<set>
#include<map>

using namespace std;

#define int long long
#define endl "\n"

constexpr long long INF = (long long)1e18;
constexpr long long MOD = 1'000'000'007; 

struct fast_io {
	fast_io(){
		std::cin.tie(nullptr);
		std::ios::sync_with_stdio(false);
	};
} fio;

signed main(){
	cout<<fixed<<setprecision(10);
	
	int N;
	int ans = INF;
	vector<int> A;
	map<int,int> mp;
	set<int> s1, s2;
	
	cin>>N;
	
	A.resize(N);
	
	for(int i = 0; i < N; i++){
		cin>>A[i];
		
		if(i){
			s2.insert(A[i]);
		}
	}
	
	for(int i = 1; i + 1 < N; i++){
		s1.insert(A[i-1]);
		s2.erase(A[i]);
		
		{
			auto it1 = s1.begin();
			auto it2 = s2.begin();
			
			if(it1 != s1.end() && it2 != s2.end() &&
				*it1 < A[i] && *it2 < A[i]
			) {
				ans = min(ans, *it1 + A[i] + *it2);
			}
		}
		
		{
			auto it1 = s1.upper_bound(A[i]);
			auto it2 = s2.upper_bound(A[i]);
			
			if(it1 != s1.end() && it2 != s2.end() &&
				*it1 > A[i] && *it2 > A[i]
			) {
				ans = min(ans, *it1 + A[i] + *it2);
			}
		}
		
	}
	
	if(ans != INF) cout<<ans<<endl;
	else cout<<-1<<endl;
	
	return 0;
}