N = int(input())
P = list(map(int, input().split()))

ans = 0
for i in range(N):
    if P[i] == i+1:
        tmp = P[i]
        P[i] = P[i+1]
        P[i+1] = tmp
        ans += 1


for i in range(N):
    if P[i] == i+1:
        ans += 1

print(ans)