#include<bits/stdc++.h>
inline void read(int&a)
{
	a=0;char c=getchar();
	while(c<'0'||c>'9')c=getchar();
	while(c>='0'&&c<='9')
	{
		a=(a<<1)+(a<<3)+c-'0';
		c=getchar();
	}
}
inline void write(int a)
{
	if(a<0){putchar('-');a=-a;}
	if(a>9)write(a/10);
	putchar(a%10+'0');
}
const int maxn=1e6+1;
int next1[maxn],dfn[maxn],low[maxn],to1[maxn],point1[maxn],from1[maxn];
int n,m,r=0,d=0;
bool vis[maxn];
inline void add1(int a,int b)
{
	next1[++r]=point1[a];
	to1[r]=b;from1[r]=a;
	point1[a]=r;
}
int in[maxn],next[maxn],to[maxn],point[maxn],from[maxn];
inline void add(int a,int b)
{
	++in[b];
	next[++d]=point[a];
	to[d]=b;
	point[a]=d;
}
int tt,f[maxn],ans=0;
int stack[maxn],belong[maxn],end=0,value[maxn];
bool in_stack[maxn];
inline void tarjan(int p)
{
	
	vis[p]=true;
	dfn[p]=low[p]=++tt;
	//value[p]=1;
	int side=point1[p];
	while(side)
	{
		if(!vis[to1[side]])
		{
			stack[++end]=to1[side];
			in_stack[to1[side]]=true;
			tarjan(to1[side]);
			low[p]=std::min(low[p],low[to1[side]]);
		}
		else if(in_stack[to1[side]])
		{
			low[p]=std::min(low[p],low[to1[side]]);
		}
		side=next1[side];
	}
	if(dfn[p]==low[p])
	{
		while(stack[end]!=p)
		{
			belong[stack[end]]=p;
			in_stack[stack[end]]=false;
			value[p]+=value[stack[end]];
			--end;
		}
		belong[p]=p;
		in_stack[p]=false;
		--end;
	}
}
inline void dfs(int p)
{
//	write(p);putchar(' ');
	int side=point[p];
	while(side)
	{
		if(f[p]+value[to[side]]>f[to[side]])
			{f[to[side]]=f[p]+value[to[side]];dfs(to[side]);}
		side=next[side];
	}
}
int main()
{
	read(n);read(m);
	memset(in,0,sizeof(in));
	memset(value,0,sizeof(value));
	memset(in_stack,false,sizeof(in_stack));
	memset(vis,false,sizeof(vis));
	memset(point1,0,sizeof(point1));
	int u,v;
	for(int i=1;i<=n;i++)value[i]=1;
	for(int i=1;i<=m;i++)
	{
		read(u);read(v);
		add1(u,v);
	}
	tt=0;
	for(int i=1;i<=n;i++)
	{
		if(!vis[i])
		{
			stack[++end]=i;in_stack[i]=true;
			tarjan(i);
		}
	}
	for(int i=1;i<=r;i++)if(belong[from1[i]]!=belong[to1[i]])add(belong[from1[i]],belong[to1[i]]);
	for(int i=1;i<=n;i++)if(!in[i]&&belong[i]==i){f[i]=value[i];dfs(i);}//puts("dd");
	for(int i=1;i<=n;i++)ans=std::max(ans,f[i]);
	write(ans);
	return 0;
}