// eddy1021
//#include <bits/stdc++.h>
#include <iostream>
#include <stdio.h>
#include <vector>
#include <queue>
using namespace std;
typedef double D;
typedef long long ll;
typedef pair<int,int> PII;
#define mod9 1000000009ll
#define mod7 1000000007ll 
#define INF 1023456789ll
#define FI first
#define SE second
#define PB push_back
#define MP make_pair
#define eps 1e-7
ll getint(){
    ll _x=0,_tmp=1; char _tc=getchar();
    while( (_tc<'0'||_tc>'9')&&_tc!='-' ) _tc=getchar();
    if( _tc == '-' ) _tc=getchar() , _tmp = -1;
    while(_tc>='0'&&_tc<='9') _x*=10,_x+=(_tc-'0'),_tc=getchar();
    return _x*_tmp;
}
ll mypow( ll _a , ll _x , ll _mod ){
    if( _x == 0 ) return 1ll;
    ll _tmp = mypow( _a , _x / 2 , _mod );
    _tmp = ( _tmp * _tmp ) % _mod;
    if( _x & 1 ) _tmp = ( _tmp * _a ) % _mod;
    return _tmp;
}
bool equal( D _x ,  D _y ){
    return _x > _y - eps && _x < _y + eps;
}
int __ = 1 , cs;
/*********default*********/
#define N 110
int n , m , indeg[ N ];
vector<int> v[ N ];
void build(){

}
void init(){
    for( int i = 1 ; i <= n ; i ++ ) v[ i ].clear();
    for( int i = 1 ; i <= n ; i ++ ) indeg[ i ] = 0;
    while( m -- ){
        int ta , tb;
        ta = getint(); tb = getint();
        v[ tb ].PB( ta );
        indeg[ ta ] ++;
    }
}
queue<int> Q;
void solve(){
    while( Q.size() ) Q.pop();
    for( int i = 1 ; i <= n ; i ++ )
        if( indeg[ i ] == 0 ) Q.push( i );
    int got = 0;
    while( Q.size() ){
        int tn = Q.front(); Q.pop(); got ++;
        for( int i = 0 ; i < (int)v[ tn ].size() ; i ++ ){
            indeg[ v[ tn ][ i ] ] --;
            if( indeg[ v[ tn ][ i ] ] == 0 )
                Q.push( v[ tn ][ i ] );
        }
    }
    puts( got == n ? "YES" : "NO" );
}
int main(){
    build();
//    __ = getint();
    while( cin >> n >> m ){
//    while( __ -- ){
        init();
        solve();
    }
}