Description：

Solution：

Code：

#include<algorithm>

#include<iostream>

#include<cstdlib>

#include<cstdio>

#include<cmath>

#include<cstring>

using namespace std;

int n,m;

#define MAXN 100

#define MAXM 1000

bool g[MAXN << 1][MAXN << 1];

struct edge

{

	int to,nxt;

}e[MAXN * MAXN * 16];

int edgenum = 0;

int lin[MAXN << 3] = {0};

void add(int a,int b)

{

	++edgenum;e[edgenum].to = b;e[edgenum].nxt = lin[a];lin[a] = edgenum;

	++edgenum;e[edgenum].to = a;e[edgenum].nxt = lin[b];lin[b] = edgenum;

	return;

}

bool v[MAXN << 2];

int match[MAXN << 2];

bool find(int k)

{

	for(int i = lin[k];i != 0;i = e[i].nxt)

	{

		if(!v[e[i].to])

		{

			v[e[i].to] = true;

			if(match[e[i].to] == -1 || find(match[e[i].to]))

			{

				match[e[i].to] = k;

				return true;

			}

		}

	}

	return false;

}

int main()

{

	scanf("%d%d",&n,&m);

	int a,b;

	memset(g,0,sizeof(g));

	for(int i = 1;i<= m;++i)

	{

		scanf("%d%d",&a,&b);

		g[a][b] = true;

	}

	for(int k = 1;k <= n;++k)

	{

		for(int i = 1;i <= n;++i)

		{

			for(int j = 1;j <= n;++j)

			{

				g[i][j] |= (g[i][k] & g[k][j]);

			}

		}

	}

	for(int i = 1;i <= n;++i)

	{

		for(int j = 1;j <= n;++j)

		{

			if(g[i][j])add(i,j + n);

		}

	}

	memset(match,-1,sizeof(match));

	int ans = 0;

	for(int i = 1;i <= n;++i)

	{

		memset(v,0,sizeof(v));

		if(find(i))++ans;

	}

	cout << (n - ans) << endl;

	return 0;

}