#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define mod 1000000007
#define esp 0.00000000001
const int N=2e3+10,M=1e6+10,inf=1e9;
int n,m;
vector<int>edge[N];
int du[N];
int main()
{
    while(~scanf("%d%d",&n,&m))
    {
        queue<int>q;
        memset(du,0,sizeof(du));
        for(int i=0;i<=n;i++)
            edge[i].clear();
        int ans=0;
        for(int i=1;i<=m;i++)
        {
            int u,v;
            scanf("%d%d",&u,&v);
            edge[u].push_back(v);
            du[v]++;
        }
        for(int i=1;i<=n;i++)
        {
            if(!du[i])q.push(i);
        }
        while(!q.empty())
        {
            int v=q.front();
            q.pop();
            ans++;
            for(int i=0;i<edge[v].size();i++)
            {
                 du[edge[v][i]]--;
                 if(!du[edge[v][i]])
                    q.push(edge[v][i]);
            }
        }
        if(ans==n)
            printf("YES\n");
        else
            printf("NO\n");
    }
    return 0;
}