#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=n-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[8]={1,0,-1,0,1,1,-1,-1};
const ll dx[8]={0,-1,0,1,1,-1,1,-1};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
int main(){
    ll n;cin >> n;
    vl v(n);rep(i,n)cin >> v[i];
    ll res=inf;
    vl cl(n),cr(n);cl[0]=v[0];cr[n-1]=v[n-1];
    rep(i,n-1){
        cl[i+1]=min(cl[i],v[i]);
    }
    for(int i=n-1;i>=1;i--){
        cr[i-1]=min(cr[i],v[i]);
    }
    for(int i=1;i<n-1;i++){
        if(cl[i]<v[i]&&cr[i]<v[i]){
            chmin(res,cl[i]+v[i]+cr[i]);
        }
    }
    rep(i,n-1){
        cl[i+1]=max(cl[i],v[i]);
    }
    for(int i=n-1;i>=1;i--){
        cr[i-1]=max(cr[i],v[i]);
    }
   /*rep(i,n)cout << cl[i]<<" ";
    cout << endl;
    rep(i,n)cout << cr[i]<<" ";
    cout << endl;*/
    for(int i=1;i<n-1;i++){
        if(cl[i]>v[i]&&cr[i]>v[i]){
            chmin(res,cl[i]+v[i]+cr[i]);
        }
    }
    if(res<inf)cout << res <<endl;
    else cout << -1 <<endl;
}