#include <map>
#include <set>
#include <list>
#include <deque>
#include <queue>
#include <cmath>
#include <stack>
#include <string>
#include <vector>
#include <cctype>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <utility>
#include <iostream>
#include <iterator>
#include <algorithm>
using namespace std;
#define REP(i, n) for(int i = 0; i < n; ++i)
#define REP1(i, a, n) for(int i = a; i <= n; ++i)
#define REPD(i, n) for(int i = n; i >= 0; --i)
bool MAP[110][110];
int indegree[110];
int n, m;
typedef long long ll;

bool topo()
{
    queue<int> que;
    REP1(i, 1, n) {
        if(indegree[i] == 0) que.push(i);
    }
    int tot = 0;
    while(!que.empty()) {
        int u = que.front(); que.pop();
        tot++;
        REP1(i, 1, n) {
            if(MAP[u][i]) {
                indegree[i]--;
                if(indegree[i] == 0) que.push(i);
            }
        }
    }
    if(tot == n) return 1;
    else return 0;
}

int main()
{
#ifdef LOCAL
freopen("in","r",stdin);
#endif
    while(~scanf("%d%d", &n, &m)) {
        memset(MAP, 0, sizeof(MAP));
        memset(indegree, 0, sizeof(indegree));
        int a, b;
        REP(i, m) {
            scanf("%d%d", &a, &b);
            if(!MAP[b][a]) indegree[a]++;
            MAP[b][a] = 1;
        }
        if(topo()) puts("YES");
        else puts("NO");
    }
    return 0;
}