n = int(input())
P = list(map(int,input().split()))
ma = n
ans = 0
use = [0]*(n+1)
score = 0
for i in range(n)[::-1]:
    p = P[i]
    if p > ma:
        ans = max(ans,score-i-1+i*(i+1)//2)
        break
    elif p == ma:
        ans = max(ans,score-i-1+i*(i+1)//2)
        use[p] = 1
    else:
        score += i+1
        use[p+1] = 1
    while use[ma]:
        ma -= 1
print(ans)