#include <algorithm>
#include <cstdio>
#include <queue>
#include <utility>
#include <vector>

using namespace std;

#define REP(i, n) for (int i = 0; i < int(n); i++)
typedef pair<int, int> PII;
typedef vector<PII> VPII;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<bool> VB;
const int INF = 12345678;

int solve(VVI &d)
{
    const int N = d.size();
    REP (k, N) REP (i, N) REP (j, N) d[i][j] = min(d[i][j], d[i][k] + d[k][j]);

#define ENCODE(p1, p2) ((p1) * N + (p2))
#define DECODE1(p) ((p) / N)
#define DECODE2(p) ((p) % N)
#define RELAX(p1, p2, c) do { const int p = ENCODE((p1), (p2)); if ((c) < cost[p]) q.push(PII(cost[p] = (c), p)); } while (0)
    priority_queue<PII, VPII, greater<PII> > q;
    VI cost(N*N, INF);
    VB searched(N*N, false);
    RELAX(0, 0, 1);
    while (!q.empty()) {
        const PII p = q.top(); q.pop();
        const int cost_u = p.first, u = p.second, u1 = DECODE1(u), u2 = DECODE2(u);
        if (searched[u]) continue;
        if (u1 == 1 && u2 == 1) return cost_u;
        REP (v, N) if (d[u1][v] != INF) RELAX(v, u2, cost_u + d[u1][v] + (v == u2 ? -1 : 0));
        REP (v, N) if (d[v][u2] != INF) RELAX(u1, v, cost_u + d[v][u2] + (v == u1 ? -1 : 0));
        if (u1 != u2 && d[u1][u2] != INF) RELAX(u2, u1, cost_u + d[u1][u2] - 1);
        searched[u] = true;
    }
    return -1;
}

int main()
{
    for (int N, M, t = 1; scanf("%d%d", &N, &M), N || M; t++) {
        VVI d(N, VI(N, INF));
        REP (i, N) d[i][i] = 0;
        REP (i, M) {
            int u, v; scanf("%d%d", &u, &v); u--, v--;
            d[u][v] = 1;
        }

        int answer = solve(d);
        printf("Network %d\n", t);
        if (answer == -1)
            printf("IMPOSSIBLE");
        else
            printf("Minimum number of nodes = %d", answer);
        printf("\n\n");
    }
    return 0;
}