N=int(input())
A=list(map(int, input().split()))
LMI=[0 for i in range(N)]
RMI=[0 for i in range(N)]
mi,ma=10**9,0
for i in range(N):
    mi=min(A[i],mi)
    LMI[i]=mi
    
mi,ma=10**9,0
for i in range(N):
    mi=min(A[-1-i],mi)
    RMI[-1-i]=mi
ans=10**9
for i in range(1,N-1):
    if LMI[i-1]<A[i] and A[i]>RMI[i+1]:
        d=LMI[i-1]+A[i]+RMI[i+1]
        ans=min(d,ans)
        
for i in range(1,N-1):
    if LMI[i-1]>A[i] and A[i]<RMI[i+1]:
        d=LMI[i-1]+A[i]+RMI[i+1]
        ans=min(d,ans)        

        
if ans==10**9:
    print(-1)
else:
    print(ans)