import itertools
def solve():
    N = int(input())
    p = list(map(int, input().split()))

    one_cnt = 0
    swap_cnt = 0
    for i in range(N):
        if p[i] == i + 1:
            one_cnt += 1
        else:
            if one_cnt % 2 == 0:
                swap_cnt += one_cnt / 2
            else:
                swap_cnt += one_cnt // 2 + 1
            one_cnt = 0
    
    print(int(swap_cnt))

if __name__ == '__main__':
    solve()
