def solution(n, graph):
    def dfs(s):
        visited = [0] * (n + 1)
        s = [s]
        while s:
            u = s.pop()
            if not visited[u]:
                visited[u] = 1
                s.extend(sorted(graph[u], reverse=True))
        return visited[1:]

    for v in range(1, n+1):
        visited = dfs(v)
        for i in visited:
            if not i:
                return False
    return True


n, m = map(int, input().split())
graph = [[] for _ in range(n+1)]
for _ in range(m):
    i, j = map(int, input().split())
    graph[i].append(j)
if solution(n, graph):
    print('YES')
else:
    print('NO')