N = int(input())
p = list(map(int,input().rstrip().split(" ")))
ans = 0
swap = 0
for i in range(N - 1):
    if p[i] == i + 1:
        swap = p[i]
        p[i] = p[i + 1]
        p[i + 1] = swap
        ans += 1
print(ans)