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

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

print(cnt)