import numpy as np

N = int(input())
A = np.array(list(map(int, input().split())))

L_min_Acc = np.minimum.accumulate(A).tolist()
R_min_Acc = np.minimum.accumulate(A[::-1])[::-1].tolist()
L_max_Acc = np.maximum.accumulate(A).tolist()
R_max_Acc = np.maximum.accumulate(A[::-1])[::-1].tolist()

ans = float('inf')
for i in range(1, N - 1):
    if A[i] > L_min_Acc[i - 1] and A[i] > R_min_Acc[i + 1]:
        ans = min(ans, L_min_Acc[i - 1] + A[i] + R_min_Acc[i + 1])
    if A[i] < L_max_Acc[i - 1] and A[i] < R_max_Acc[i + 1]:
        ans = min(ans, L_max_Acc[i - 1] + A[i] + R_max_Acc[i + 1])
print(ans if ans != float('inf') else -1)