#include <cstdio>
#define N 100 + 5
#define M 10000 + 5

int n, m, tot;
int Head[N], Deg[N], q[N];

struct Edge
{
    int next, node;
}h[M];

inline void addedge(int u, int v)
{
    h[++ tot].next = Head[u];
    Head[u] = tot;
    h[tot].node = v;
}

int main()
{
    while (scanf("%d%d", &n, &m) == 2)
    {
        tot = 0;
        for (int i = 1; i <= n; i ++)
            Deg[i] = Head[i] = 0;
        for (int i = 1, u, v; i <= m; i ++)
        {
            scanf("%d%d", &u, &v);
            addedge(u, v);
            Deg[v] ++;
        }
        int l = 1, r = 0;
        for (int i = 1; i <= n; i ++)
            if (!Deg[i]) q[++ r] = i;
        while (l <= r)
        {
            int z = q[l ++];
            for (int i = Head[z]; i; i = h[i].next)
            {
                int d = h[i].node;
                Deg[d] --;
                if (!Deg[d]) q[++ r] = d;
            }
        }
        if (r == n) puts("YES");
            else puts("NO");
    }
    
    return 0;
}