from itertools import accumulate

N = int(input())

A = list(map(int,input().split()))
l = list(accumulate(A, min))
r = list(accumulate(A[::-1], min))[::-1]
INF = 10**18
ans = INF

for i in range(1,N-1):
  a,b,c = l[i-1],A[i],r[i+1]
  if a <= b and c <= b:
    ans = min(ans,a+b+c)
  if a >= b and c >= b:
    ans = min(ans,a+b+c)

if ans == INF:
  print(-1)
else:
  print(ans)