N = int(input())
A = list(map(int,input().split()))
INF = 10**10
L = [INF]*N
R = [INF]*N
L[0] = A[0]
R[N-1] = A[N-1]
for i in range(1,N):
    L[i] = min(A[i],L[i-1])
    R[N-1-i] = min(A[N-1-i],R[N-i])

ans = INF
for i in range(1,N-1):
    if L[i-1] < A[i] and R[i+1] < A[i]:
        ans = min(ans,L[i-1] + A[i] + R[i+1])
        
A = [-i for i in A]
L = [INF]*N
R = [INF]*N
L[0] = A[0]
R[N-1] = A[N-1]
for i in range(1,N):
    L[i] = min(A[i],L[i-1])
    R[N-1-i] = min(A[N-1-i],R[N-i])

tans = -INF

for i in range(1,N-1):
    if L[i-1] < A[i] and R[i+1] < A[i]:
        tans = max(tans,L[i-1] + A[i] + R[i+1])
tans *= -1
ans = min(tans,ans)
if ans == INF:
    print(-1)
else:
    print(ans)