import heapq

ans=1<<60;n=int(input())
a=list(map(int,input().split()))
hq1=[];hq2=[]
for i in range(1,n):
    heapq.heappush(hq1,(a[i],i))
    heapq.heappush(hq2,(-a[i],i))
c=a[0];d=a[0]
for i in range(1,n-1):
    if a[i]>c and a[i]>hq1[0][0]:
        ans=min(ans,a[i]+c+hq1[0][0])
    if a[i]<d and a[i]<-hq2[0][0]:
        ans=min(ans,a[i]+d+-hq2[0][0])
    if hq1[0][1]==i:
        heapq.heappop(hq1)
    if hq2[0][1]==i:
        heapq.heappop(hq2)
if ans==1<<60:
    ans=-1
print(ans)