import sys
sys.setrecursionlimit(10000)
input()
l=[int(i) for i in input().split()]
d = {}
def dp(i,j):
    if (i,j) in d:
        return d[(i,j)]
    if j-i==1:
        return l[i]
    a=dp(i,j-1)
    b=dp(i+1,j)
    if a!=b:
        v = max(a,b)
        d[(i,j)] = v
        return v
    lo=i+1
    hi=j
    while lo < hi:
        m=(hi+lo)//2
        e=dp(i,m)
        f=dp(m,j)
        if e<a and f<a:
            d[(i,j)] = a
            return a
        if e==a and f==a:
            d[(i,j)] = 2*a
            return 2*a
        if e<a:
            lo=m+1
        else:
            hi=m
    d[(i,j)] = a
    return a
print(dp(0,len(l)))