N = int(input())

#初項 a，公差 1，末項 l
l = N
a = 1

wa =  (l - a + 1) * (a + l) // 2

A = list(map(int, input().split()))
idx = A.index(N) + 1

takeda = wa - idx
suzuki = idx

ans = max(takeda - suzuki, 0)
#print(N, ans)

for i in range(N - 1, 0, -1):
    idx = A.index(i) + 1
    takeda -= idx
    suzuki = idx
    #print(i, takeda, suzuki, takeda - suzuki)
    if max(takeda - suzuki, 0) > ans:
        ans = max(takeda - suzuki, 0)
    if ans > takeda:
        break
print(ans)