def resolve():
    n = int(input())
    p = list(map(int, input().split()))
    ans = 0
    for i in range(n - 1):
        if i + 1 == p[i]:
            p[i], p[i + 1] = p[i + 1], p[i]
            ans += 1
    print(ans)


if __name__ == "__main__":
    resolve()
