queue<int> node;

for (int i = 0; i < n; ++i) {

if (inDegree[i] == 0) {

node.push(i);

}

}

int number = 0; //拓扑序列结点个数

while (!node.empty()) {

int u = node.front();

node.pop();

number++; //拓扑序列结点加一

for (int i = 0; i < graph[u].size(); ++i) {

int v = graph[u][i];

inDegree[v]--; //后继结点入度减一

if (inDegree[v] == 0) {

node.push(v);

}

}

}

return n == number; //判断能否产生拓扑排序

int n, m;

while (scanf("%d%d", &n, &m) != EOF) {

if (n == 0 && m == 0) {

break;

}

memset(graph, 0, sizeof(graph));

memset(inDegree, 0, sizeof(inDegree));

while (m--) {

int from, to;

scanf("%d%d", &from, &to);

graph[from].push_back(to);

inDegree[to]++;

}

if (TopologicalSort(n)) {

printf("YES\n");

} else {

printf("NO\n");

}

}

return 0;