def swap(a, b):
    tmp = a
    a = b
    b = tmp
    return a, b


def solve(): # 072D- Derangement
    N = int(input())
    p = list(map(int, input().split(" ")))
    cnt = 0

    for i in range(1, N):
        if i == p[i-1]:
            p[i], p[i+1] = swap(p[i], p[i+1])
            cnt += 1
        elif i == N:
            p[i-1], p[i] = swap(p[i-1], p[i])
            cnt += 1
        else:
            pass

    print(cnt + 1)
    

solve()