#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define rep(i,n) for(ll i=0;i<(n);i++)
#define pll pair<ll,ll>
#define pq priority_queue
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define ios ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define lb(c,x) distance(c.begin(),lower_bound(all(c),x))
#define ub(c,x) distance(c.begin(),upper_bound(all(c),x))

using namespace std;

template<class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}

const ll INF=1e18;

int main(){
    ll n;
    cin >> n;
    vector<ll> a(n),maxle(n),maxri(n),minle(n),minri(n);
    rep(i,n){
        cin >> a[i];
    }
    ll maxr=-1,maxl=-1,minr=INF,minl=INF;
    rep(i,n){
        maxle[i]=maxl;
        maxl=max(maxl,a[i]);
        minle[i]=minl;
        minl=min(minl,a[i]);
    }
    for(ll i=n-1;i>=0;i--){
        maxri[i]=maxr;
        maxr=max(maxr,a[i]);
        minri[i]=minr;
        minr=min(minr,a[i]);
    }
    ll ans=INF;
    for(ll i=1;i<n-1;i++){
        if(maxle[i]>a[i] && maxri[i]>a[i]){
            ans=min(ans,maxle[i]+maxri[i]+a[i]);
        }
        if(minle[i]<a[i] && minri[i]<a[i]){
            ans=min(ans,minle[i]+minri[i]+a[i]);
        }
    }
    if(ans==INF) cout << -1 << endl;
    else cout << ans << endl;
}