1 #include <iostream>
 2 #include <cstring>
 3 #include <cstdio>
 4 using namespace std;
 5
 6 const int maxn = 100 + 10;
 7 int map[maxn][maxn];
 8 int in[maxn];
 9 int n, m;
10
11 int main()
12 {
13     #ifndef ONLINE_JUDGE
14         freopen("in.txt", "r", stdin);
15     #endif // ONLINE_JUDGE
16     int from, to;
17     while (scanf("%d%d", &n, &m) != EOF) {
18         memset(map, 0, sizeof(map));
19         memset(in, 0, sizeof(in));
20         for (int i = 0; i < m; i++) {
21             scanf("%d%d", &to, &from);
22             if (!map[from][to])
23             {
24                 map[from][to] = 1;
25                 in[to]++;       // 入度
26             }
27         }
28         for (int i = 1; i <= n; i++)
29         {
30             for (int j = 1; j <= n; j++)
31             {
32                 if (!in[j])
33                 {
34                     in[j] = -1;
35                     for (int k = 1; k <= n; k++)
36                     {
37                         if (map[j][k])
38                         {
39                             map[j][k] = 0;
40                             in[k]--;
41                         }
42                     }
43                     break;
44                 }
45             }
46         }
47         bool flag = true;
48         for (int i = 1; i <= n; i++)
49         {
50             if (in[i] != -1)
51             {
52                 flag = false;
53                 break;
54             }
55         }
56         printf("%s\n", flag ? "YES" : "NO");
57     }
58     return 0;
59 }