#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <string>
#include <cmath>
#include <map>
#include <queue>
#include <set>
using namespace std;
int st[110] , to[10010] , nt[10010] , mk[110];
int main()
{
    int n , m;
    while( scanf( "%d %d" , &n , &m ) != EOF )
    {
        memset( st , -1 , sizeof(st) );
        memset( mk , 0 , sizeof(mk) );
        int a , b , p = 0;
        for( int i=0 ; i<m ; i++ )
        {
            scanf( "%d %d" , &b , &a );
            to[p] = b; nt[p] = st[a]; st[a] = p++;
            mk[b]++;
        }
        p = 0;
        while(1)
        {
            int nw = -1;
            for( int i=1 ; i<=n ; i++ )
                if( mk[i] == 0 )
                {
                    nw = i; break;
                }
            if( nw < 0 ) break;
            for( int i=st[nw] ; i!=-1 ; i=nt[i] ) mk[ to[i] ]--;
            p++;
            mk[nw] = -1;
        }
        if( p >= n - 1 ) puts( "YES" );
        else puts( "NO" );
    }
    return 0;
}