N = int(input())
A = list(map(int,input().split()))
INF = float('inf')
minl = [INF]
for a in A:
    minl.append(min(minl[-1], a))
minr = [INF]
for a in A[::-1]:
    minr.append(min(minr[-1], a))
minr.reverse()

ans = INF
for l,a,r in zip(minl[1:], A[1:-1], minr[2:]):
    if l > a and a < r or l < a and a > r:
        ans = min(ans, l+a+r)
print(-1 if ans==INF else ans)