n = int(input())
p = list(map(int,input().split()))

ans = 0
for i in range(n - 1):
    if p[i] == i + 1:
        if p[i] == n:
            p[i - 1], p[i] = p[i], p[i - 1]
        else:
            p[i + 1], p[i] = p[i], p[i + 1]
        ans += 1

print(ans)