#include <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
#include <iomanip> // setprecision
#include <complex> // complex
#include <math.h> 
#include <climits> 
#include <assert.h>
#define rep(i,n) for(int i=0; i<(n); i++)
using namespace std;
using ll = long long;
using P = pair<int,int>;
constexpr ll INF = 1e18;
constexpr int inf = 1e9;
constexpr ll mod = 1000000007;
constexpr ll mod2 = 998244353;
const double PI = 3.1415926535897932384626433832795028841971;
const int dx[8] = {1, 0, -1, 0,1,1,-1,-1};
const int dy[8] = {0, 1, 0, -1,1,-1,1,-1};
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; }
// ---------------------------------------------------------------------------


int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N;
    cin >> N;
    vector<int> A(N);
    for(int i=0; i<N; i++) cin >> A[i];
    vector<int> mn_l(N,inf);
    vector<int> mx_l(N,0);
    mn_l[0] = A[0];
    mx_l[0] = A[0];
    for(int i=1; i<N; i++){
        mn_l[i] = min(A[i],mn_l[i-1]);
        mx_l[i] = max(A[i],mx_l[i-1]);
    }
    vector<int> mn_r(N,inf);
    vector<int> mx_r(N,0);
    mn_r[N-1] = A[N-1];
    mx_r[N-1] = A[N-1];
    for(int i=N-2; i>=0; i--){
        mn_r[i] = min(A[i],mn_r[i+1]);
        mx_r[i] = max(A[i],mx_r[i+1]);
    }
    int ans = inf;
    for(int i=1; i<N-1; i++){
        if(mn_l[i-1] < A[i] && mn_r[i+1] < A[i]){
            chmin(ans,mn_l[i-1]+mn_r[i+1]+A[i]);
        }
        if(mx_l[i-1] > A[i] && mx_r[i+1] > A[i]){
            chmin(ans,mx_l[i-1]+A[i]+mx_r[i+1]);
        }
    }
    if(ans == inf){
        cout << -1 << "\n";
    }else{
        cout << ans << "\n";
    }
    return 0;
}