#include "bits/stdc++.h"
using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define REP1(i, n) for(int i=1; i<=(n); i++)
#define RREP1(i, n) for(int i=(n); i>=1; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define out(...) printf(__VA_ARGS__)
#if DEBUG
#define debug(...) printf(__VA_ARGS__)
#else
#define debug(...) /* ... */
#endif
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

void solve();
signed main()
{
#if DEBUG
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
#endif
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}

/*================================*/
#if DEBUG
#define SIZE 100
#else
#define SIZE 223450
#endif

int N,A[SIZE];

bool is_kadomatsu(int a, int b, int c) {
    if (a==b || b == c || c==a) return false;
    if (b < a && b < c) return true;
    if (b > a && b > c) return true;
    return false;
}

template<typename T> class ST {
    using F = function<T(T, T)>;
    int size;
    T init;
    F f;
    vector<T> seg;
    int depth = 0;
    
public: ST(int size, T init, F f): size(size), init(init), f(f) {
    while (1<<depth < size) depth++;
    seg.assign(2<<depth, init);
}
    
    void update(int k, T t) {
        k += 1 << depth;
        seg[k] = t;
        while(k>>=1) seg[k] = f(seg[2*k], seg[2*k+1]);
    }
    
    T query(int a, int b) {
        a += 1<<depth;
        b += 1<<depth;
        T l = init;
        T r = init;
        while(a < b) {
            if (a & 1) l = f(l, seg[a++]);
            if (b & 1) r = f(r, seg[--b]);
            a >>= 1;
            b >>= 1;
        }
        return f(l, r);
    }
    
    T operator[] (int k) {
        return seg[k + (1<<depth)];
    }
};

ST<int> *minst, *maxst;

void solve() {
    cin>>N;
    minst = new ST<int>(N, 1e9, [](int a, int b){ return min(a,b); });
    maxst = new ST<int>(N, 0, [](int a, int b){ return max(a,b); });
    REP(i,N)cin>>A[i];
    REP(i,N) {
        minst->update(i, A[i]);
        maxst->update(i, A[i]);
    }
    int ret = 1e9;
    REP1(i,N-2) {
        {
            int a = minst->query(0, i);
            int b = minst->query(i+1, N);
            if (is_kadomatsu(a, A[i], b)) {
                debug("ok: %lld %lld %lld\n",a,A[i],b);
                chmin(ret, a+A[i]+b);
            }
        }
        {
            int a = maxst->query(0, i);
            int b = maxst->query(i+1, N);
            if (is_kadomatsu(a, A[i], b)) {
                debug("ok: %lld %lld %lld\n",a,A[i],b);
                chmin(ret, a+A[i]+b);
            }
        }
    }
    if (ret==1e9) ret=-1;
    cout << ret << endl;
}