有向图的完全可达性

'''
def dfs(num,i,visit):
    for k in num[i]:
        if visit[k] ==False:
            visit[k] = True
            dfs(num,k,visit)
'''
import collections
path = set()

def bfs(i,graph):
    global path
    que = collections.deque([i])
    while que:
        n = que.popleft()
        path.add(n)
        for j in graph[n]:
            que.append(j)
        graph[n] = []
    return 





def main():
    n,m = map(int,input().split())
    num = [[] for i in range(n+1)]
    for i in range(m):
        a,b = map(int,input().split())
        num[a].append(b)
    bfs(1,num)
    if path == {i for i in range(1,n+1)}: # 不能用() 只能用{}，圆括号用于生成元组，花括号用于生成集合
        return 1
    return -1
    '''
    visit = [False]*(n+1)
    visit[1] = True
    dfs(num,1,visit)
    for i in range(1,n+1):
        if visit[i] == False:
            return -1
    return 1
    '''
if __name__ == '__main__':
    print(main())

105. 有向图的完全可达性