#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pi (4*atan(1.0))
#define eps 1e-14
#define bug(x) cout<<"bug"<<x<<" "<<endl;
const int N=2e2+10,M=1e6+10,inf=2e9+10,mod=1e9+7;
const ll INF=1e18+10;
int n,m;
int mp[N][N];
int linker[N];
bool used[N];
bool dfs(int a)
{
    for(int i=0;i<n;i++)
      if(mp[a][i]&&!used[i])
      {
          used[i]=true;
          if(linker[i]==-1||dfs(linker[i]))
          {
              linker[i]=a;
              return true;
          }
      }
      return false;
}
int hungary()
{
    int result=0;
    memset(linker,-1,sizeof(linker));
    for(int i=0;i<n;i++)
    {
        memset(used,0,sizeof(used));
        if(dfs(i))  result++;
    }
    return result;
}
queue<int>q;
int color[N];
int bfs(int s)
{
    while(!q.empty())q.pop();
    q.push(s);
    while(!q.empty())
    {
        int x=q.front();
        q.pop();
        for(int i=0;i<n;i++)
        {
            if(mp[x][i])
            {
                if(color[i]==-1)
                    color[i]=color[x]^1,q.push(i);
                else if(color[i]==color[x])
                    return 0;
            }
        }
    }
    return 1;
}
int check()
{
    memset(color,-1,sizeof(color));
    for(int i=0;i<n;i++)
    {
        if(color[i]==-1)
        {
            color[i]=1;
            if(!bfs(i))return 0;
        }
    }
    return 1;
}
int main()
{
    while(~scanf("%d%d",&n,&m))
    {
        memset(mp,0,sizeof(mp));
        for(int i=1;i<=m;i++)
        {
            int u,v;
            scanf("%d%d",&u,&v);
            mp[u-1][v-1]=1;
        }
        if(!check())printf("No\n");
        else
        {
            int cnt=hungary();
            printf("%d\n",cnt);
        }
    }
    return 0;
}