#include<iostream>
#include<stdio.h>
//#include <bits/stdc++.h>
#include<vector>
#include<float.h>
#include<iomanip>
#include<algorithm>
#include<string>
#include<cstring>
#include<math.h>
#include<cmath>
#include<sstream>
#include<set>
#include<map>
#include<queue>
#include<cassert>
#include<cmath>
#include<cstdint>

#define INF 1e18
#define rep(i,n)for(int i=0;(i)<(int)(n);i++)
#define REP(i,a,b)for(int i=(int)(a);(i)<=(int)(b);i++)
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define vec(type,n) vector<type>(n)
#define vvec(m,n) vector<vector<int>> (int(m),vector<int>(n))
#define ALL(a)  (a).begin(),(a).end()

using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;
using P = pair<ll, ll>;

const ll MOD = INF + 7;



int main(){
   int n;
   cin >> n;
   ll res = INF;
   vector<ll> a(n), sa(n);
   
   bool f = true;
   int up = 0, down = 0;
   rep(i, n){
      cin >> a[i];
      if(i == 0)continue;
      if(a[i - 1] > a[i])down++;
      if(a[i - 1] < a[i])up++;
   }
   if(down == 0 || up == 0){
      cout << -1 << endl;
      return 0;
   }



   vector<ll> max_l(n), max_r(n), min_l(n), min_r(n);
   rep(i, n){
      if(i == 0){
         //max_l[i] = a[i];
         min_l[i] = a[i];
      }
      else {
         //max_l[i] = max(max_l[i - 1], a[i]);
         min_l[i] = min(min_l[i - 1], a[i]);
      }
   }
   reverse(ALL(a));
   rep(i, n){
      if(i == 0){
         //max_r[i] = a[i];
         min_r[i] = a[i];
      }
      else {
         //max_r[i] = max(max_r[i - 1], a[i]);
         min_r[i] = min(min_r[i - 1], a[i]);
      }
   }

   reverse(ALL(a));
   REP(i, 1, n - 2){
      ll am = INF, aM = INF;
      if(a[i] > min_l[i - 1] && a[i] > min_r[n - i - 2])am = a[i] + min_l[i - 1] + min_r[n - i - 2];
      //if(a[i] < max_l[i - 1] && a[i] < max_r[n - i - 1])aM = a[i] + max_l[i - 1] + max_r[n - i - 1];
      res = min({res, am, aM});
   }
   cout << res << endl;
}