N = int(input())
P = list(map(int,input().split()))
for i in reversed(range(N)):
    if P[i] == i + 1:
        P.pop()
        N -= 1
if P:
    sumn = N * (N + 1) // 2
    cp = P.index(N) + 1
    print(sumn - cp * 2)
else:
    print(0)