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

flag = True
for i in range(n - 1):
    if A[i] < A[i + 1]:
        flag = False
        break
if flag:
    print(-1)
    exit()

flag = True
for i in range(n - 1):
    if A[i] > A[i + 1]:
        flag = False
        break
if flag:
    print(-1)
    exit()

Lmin = [A[0] for _ in range(n)]
Rmin = [A[-1] for _ in range(n)]
for i in range(1, n):
    Lmin[i] = min(Lmin[i - 1], A[i])
for i in range(n - 2, 0, -1):
    Rmin[i] = min(Rmin[i + 1], A[i])

ans = 1 << 60
for i in range(1, n - 1):
    if Lmin[i - 1] < A[i] > Rmin[i + 1] or Lmin[i - 1] > A[i] < Rmin[i + 1]:
        ans = min(ans, Lmin[i - 1] + A[i] + Rmin[i + 1])
print(ans)