#include<stdio.h>
#include<string.h>
#include<math.h>
#include<vector>
#include<algorithm>
using namespace std;
const int maxn = 111;
int rudu[maxn];
vector<int>ljb[maxn];

int main()
{
    int n, m, u, v, ii,i;
    while (~scanf("%d%d", &n, &m))
    {
        memset(rudu, 0, sizeof(rudu));
        for (i = 0; i <= n; i++) ljb[i].clear();
        for (i = 0; i < m; i++)
        {
            scanf("%d%d", &u, &v);
            ljb[v].push_back(u);
            rudu[u]++;
        }
        int tot = 0;
        while (1)
        {
            int flag = 0;
            for (ii = 1; ii <= n; ii++)
            {
                if (rudu[ii] == 0)
                {
                    flag = 1; tot++;
                    rudu[ii]--;
                    for (i = 0; i < ljb[ii].size(); i++)
                    {
                        rudu[ljb[ii][i]]--;
                    }
                    break;
                }
            }
            if (!flag) break;
        }
        if (tot == n) printf("YES\n");
        else printf("NO\n");
    }
    return 0;
}