INF = 10 ** 9
n = int(input())
a = list(map(int, input().split()))
p = [INF] * (n + 1)
q = [INF] * (n + 1)
for i in range(n):
    p[i + 1] = min(p[i], a[i])
for i in range(n, 0, -1):
    q[i - 1] = min(q[i], a[i - 1])
ans = INF
for i in range(1, n):
    if max(p[i], q[i + 1]) < a[i]:
        ans = min(ans, p[i] + q[i + 1] + a[i])

p = [0] * (n + 1)
q = [0] * (n + 1)
for i in range(n):
    p[i + 1] = max(p[i], a[i])
for i in range(n, 0, -1):
    q[i - 1] = max(q[i], a[i - 1])
for i in range(1, n - 1):
    if min(p[i], q[i + 1]) > a[i]:
        ans = min(ans, p[i] + q[i + 1] + a[i])
if ans == INF:
    print(-1)
else:
    print(ans)