class BinaryIndexedTree:
    def __init__(self,n,default = 10**9):
        self.s = [default]*(n+1)
        self.n = n
    # idxは0-indexted
    def reset(self,val,idx):
        idx += 1
        while idx < self.n+1:
            self.s[idx] = min(self.s[idx], val)
            idx += idx&(-idx)
        return
    
    def get(self,idx):
        idx += 1
        res = 10**9
        while idx > 0:
            res = min(res, self.s[idx])
            idx -= idx&(-idx)
        return res


n = int(input())
A = list(map(int, input().split()))

mod = 10**9 + 7

Rm = [[0]*n,[0]*n]
B = sorted(range(n),key = lambda x:A[x])
ans = 10**9
for b in B:
    left = Rm[0][b]
    right = Rm[1][b]
    if left*right > 0:
        ans = min(ans, left + right + A[b])
    idx = b
    while idx < n:
        if Rm[0][idx] != 0:break
        Rm[0][idx] = A[b]
        idx += 1
    idx = b
    while idx >= 0:
        if Rm[1][idx] != 0:break
        Rm[1][idx] = A[b]
        idx -= 1
RM = [BinaryIndexedTree(n), BinaryIndexedTree(n)]
for b in B[::-1]:
    left = RM[0].get(b)
    right = RM[1].get(n-b-1)
    if (10**9-left)*(10**9-right) > 0:
        ans = min(ans, left + right + A[b])
    RM[0].reset(A[b], b)
    RM[1].reset(A[b], n-b-1)
print(ans if ans != 10**9 else -1)