#Uses python3

import sys
import queue

def BFS(adj, s):
    dist = [-1] * len(adj)
    dist[s] = 0
    q = queue.Queue()
    q.put(s)
    while not q.empty():
        u = q.get()
        for v in adj[u]:
            if dist[v] == -1:
                q.put(v)
                dist[v] = dist[u] + 1
    return dist

def distance(adj, s, t):
    dist = BFS(adj, s)
    return dist[t]

if __name__ == '__main__':
    #input = sys.stdin.read()
    with open('test','r') as f:
        input = f.read()
    data = list(map(int, input.split()))
    n, m = data[0:2]
    data = data[2:]
    edges = list(zip(data[0:(2 * m):2], data[1:(2 * m):2]))
    adj = [[] for _ in range(n)]
    for (a, b) in edges:
        adj[a - 1].append(b - 1)
        adj[b - 1].append(a - 1)
    s, t = data[2 * m] - 1, data[2 * m + 1] - 1
    print(distance(adj, s, t))

Create bfs.py