from itertools import combinations
from collections import defaultdict


def dfs(st, end, valid):
    todo = [st]
    vis = set([st])
    while len(todo) > 0:
        cur = todo.pop()
        if cur == end:
            return True
        vis.add(cur)
        for ne in graph[cur]:
            if ne in valid and ne not in vis:
                todo.append(ne)
                vis.add(ne)
    return False


case = 0
while True:
    case += 1
    n, m = map(int, input().split())
    if n == m == 0:
        break

    graph = defaultdict(list)

    for _ in range(m):
        x, y = map(int, input().split())
        graph[x - 1].append(y - 1)
    all_ = set(range(n))
    if not dfs(0, 1, all_) or not dfs(1, 0, all_):
        print("Network {}\nIMPOSSIBLE\n".format(case))
        continue

    number = 0
    res = -1
    while True:
        for psol in combinations(range(n), number):
            val = set(psol) | {0, 1}
            if dfs(0, 1, val) and dfs(1, 0, val):
                res = number + 2
                break
        if res != -1:
            break
        number += 1
    print("Network {}\nMinimum number of nodes = {}\n".format(case, res))