import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians, gcd, log2
from itertools import accumulate, permutations, combinations, product
from operator import itemgetter, mul, add
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left
from heapq import heappush, heappop
from functools import reduce, lru_cache
def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
def ZIP(n): return zip(*(MAP() for _ in range(n)))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7

class SegmentTree:
    """Segment Tree (Point Update & Range Query)
    Query
        1. update(i, val): update i-th value(0-indexed) to val
        2. query(low, high): find f(value) in [low, high)
    Complexity
        time complexity: O(log n)
        space complexity: O(n)
    """
    def __init__(self, N, f, default):
        self.N = 1 << (N-1).bit_length()
        self.default = default
        self.f = f
        self.segtree = [self.default] * ((self.N << 1) - 1)
    @classmethod
    def create_from_array(cls, arr, f, default):
        N = len(arr)
        self = cls(N, f, default)
        for i in range(N):
            self.segtree[self.N - 1 + i] = arr[i]
        for i in reversed(range(self.N - 1)):
            self.segtree[i] = self.f(
                self.segtree[(i << 1) + 1], self.segtree[(i << 1) + 2])
        return self
    def update(self, i, val):  # update
        i += self.N - 1
        self.segtree[i] = val
        while i > 0:
            i = (i - 1) >> 1
            self.segtree[i] = self.f(
                self.segtree[(i << 1)+1], self.segtree[(i << 1)+2])
    def query(self, low, high):  # query
        # query [l, r)
        low, high = low + self.N, high + self.N
        ret = self.default
        while low < high:
            if low & 1:
                ret = self.f(ret, self.segtree[low-1])
                low += 1
            if high & 1:
                high -= 1
                ret = self.f(ret, self.segtree[high-1])
            low, high = low >> 1, high >> 1
        return ret
    def get(self, k):  # get k-th value(0-indexed)
        return self.segtree[k+self.N-1]
    def all(self):  # all range query
        return self.segtree[0]

N = INT()
A = LIST()

segtree = SegmentTree.create_from_array(A, min, 10**9)

ans = 10**10
for i in range(1, N-1):
    l = segtree.query(0, i)
    r = segtree.query(i, N)
    if l < A[i] > r or l > A[i] < r:
        ans = min(ans, A[i]+l+r)
    
print(ans if ans != 10**10 else -1)