#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <queue>
using namespace std;
const int maxn = 105;
const int maxm = 10005;

int n, m, id, done;
int in[maxn], head[maxn], next[maxm], e[maxm];
bool vis[maxn];

inline void add_edge(const int &x, const int &y)
{
    next[++id] = head[x];
    head[x] = id;
    e[id] = y;
}

bool bfs()
{
    queue<int> q;
    int i, now;
    for (i = 1; i <= n; ++i)
        if (!in[i])
        {
            q.push(i);
            vis[i] = true;
            ++done;
        }
    if (done == n) return true;
    while (!q.empty())
    {
        now = q.front(); q.pop();
        for (i = head[now]; i != -1; i = next[i])
        {
            int &v(e[i]);
            if (!vis[v] && !(--in[v]))
            {
                q.push(v);
                vis[v] = true;
                if (++done == n) return true;
            }
        }
    }
    return false;
}

int main()
{
    int i, x, y;
    while (scanf("%d%d", &n, &m) == 2)
    {
        id = done = 0;
        memset(in, 0, sizeof in);
        memset(head, -1, sizeof head);
        memset(next, -1, sizeof next);
        memset(vis, 0, sizeof vis);
        for (i = 1; i <= m; ++i)
        {
            scanf("%d%d", &x, &y);
            ++in[x];
            add_edge(y, x);
        }
        puts(bfs() ? "YES" : "NO");
    }
    return 0;
}