백준 18352 특정 거리의 도시 찾기

import heapq
INF = int(1e9)

n, m, k, x = map(int, input().split())

g = [[] for _ in range(n+1)]

for _ in range(m):
    a, b = map(int, input().split())
    g[a].append(b)

d = [INF] * (n+1)

def dijkstra(start):
    q = []
    heapq.heappush(q, (0, start))
    d[start] = 0
    while q:
        dist, now = heapq.heappop(q)
        if d[now] < dist:
            continue
        for i in g[now]:
            cost = dist + 1
            if cost < d[i]:
                d[i] = cost
                heapq.heappush(q, (cost, i))

dijkstra(x)

ans = 1
for i in range(1, n+1):
    if d[i] == k:
        print(i)
        ans = 0
if ans:
    print(-1)

다익스트라로 해결, pypy 제출