N = int(input())
A = list(map(int, input().split()))
S = [0] * (N+1)
S[-1] = float('inf')
for i, a in enumerate(A[::-1]):
    S[N-1-i] = min(a, S[N - i])

ans = float('inf')
first = A[0]
Done = set()
for i, a in enumerate(A[1:-1]):
    if (first - a) * (S[i+1] - a) > 0:
        ans = min(ans, first + S[i+1] + a)
    if first > a:
        first = a


if ans == float('inf'):
    ans = -1
print(ans)