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
print(ans if ans != 10**9 else -1)