N = int(input())
A = list(map(int,input().split()))
for i in range(N-1):
    if A[i]<A[i+1]:
        break
else:
    print(-1)
    exit()
for i in range(N-1):
    if A[i]>A[i+1]:
        break
else:
    print(-1)
    exit()
LMIN = [A[0]]*N
RMIN = [A[-1]]*N
for i in range(1,N):
    LMIN[i] = min(LMIN[i-1],A[i])
for i in reversed(range(N-1)):
    RMIN[i] = min(A[i],RMIN[i+1])
ans = 10**10
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)