import sys
input=sys.stdin.readline
def I(): return int(input())
def MI(): return map(int, input().split())
def LI(): return list(map(int, input().split()))

def main():
    mod=10**9+7
    N=I()
    A=LI()
    inf=10**9
    ml=[inf]*N
    mr=[inf]*N
    
    m=inf
    for i in range(N):
        ml[i]=m
        if A[i]<m:
            m=A[i]
        
    m=inf
    for i in range(N-1,-1,-1):
        mr[i]=m
        if A[i]<m:
            m=A[i]
        
    ans=inf
    for i in range(N):
        if ml[i]<A[i] and mr[i]<A[i]:
            temp=ml[i]+mr[i]+A[i]
            ans=min(ans,temp)
            
    ###################
    
    for i in range(N):
        A[i]*=-1        
    inf=-1*(10**9)
    ml=[inf]*N
    mr=[inf]*N
    
    m=inf
    for i in range(N):
        ml[i]=m
        if A[i]>m:
            m=A[i]
        
    m=inf
    for i in range(N-1,-1,-1):
        mr[i]=m
        if A[i]>m:
            m=A[i]
        
    ans2=inf
    for i in range(N):
        if ml[i]<A[i] and mr[i]<A[i]:
            temp=ml[i]+mr[i]+A[i]
            ans2=max(ans2,temp)
            
    ans2*=-1
    # print(ml)
    # print(mr)
    # print(ans2)
    
    ans=min(ans,ans2)
    inf=10**9
    
    if ans>=inf:
        ans=-1
    print(ans)
    


main()