// Google Code Jam 2022 World Finals
// Problem C. Slide Parade
#include <bits/stdc++.h>

using LL = long long;
using ULL = unsigned LL;

constexpr ULL E(long double x, int y) {
    while (y--) x *= 10;
    return x;
}
constexpr ULL SIZE(long double x, int y) {return E(x, y) + 5;}

const int inf32 = E(1, 9) + 5;
const LL inf64 = E(1, 18) + 5;

bool vis[205];
int n, m, mat[205];
std::pair<int, int> edge[5005];
std::vector<int> graph[205];
std::vector<int> _graph[205];
std::vector<int> ans;

bool Hungarian(int u) {
    if (vis[u]) return false;
    vis[u] = true;
    for (int v : graph[u]) {
        if (mat[v] && !Hungarian(mat[v])) continue;
        mat[v] = u;
        return true;
    }
    return false;
}
bool Dfs(int u, int s) {
    if (vis[u]) return false;
    vis[u] = true;
    if (mat[u] == s) return true;
    int w = mat[u];
    for (int v : graph[w]) {
        if (!Dfs(v, s)) continue;
        mat[v] = w;
        mat[u] = s;
        return true;
    }
    return false;
}

void Euler(int u) {
    ans.push_back(u);
    if (_graph[u].empty()) return;
    int v = _graph[u].back();
    _graph[u].pop_back();
    Euler(v);
}

void Solve(int data) {
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= n; i++) {
        mat[i] = 0;
        graph[i].clear();
        _graph[i].clear();
    }
    for (int i = 1; i <= m; i++) {
        int x, y;
        scanf("%d%d", &x, &y);
        edge[i] = {x, y};
        graph[x].push_back(y);
    }
    int cnt = 0;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) vis[j] = false;
        cnt += Hungarian(i);
    }
    if (cnt < n) {
        printf("Case #%d: IMPOSSIBLE\n", data);
        return;
    }
    for (int i = 1; i <= m; i++) {
        for (int j = 1; j <= n; j++) vis[j] = false;
        if (!Dfs(edge[i].second, edge[i].first)) {
            printf("Case #%d: IMPOSSIBLE\n", data);
            return;
        }
        for (int j = 1; j <= n; j++)
            _graph[mat[j]].push_back(j);
    }
    ans.clear();
    Euler(1);
    printf("Case #%d: %llu\n", data, ans.size());
    for (int x : ans) printf("%d ", x);
    putchar('\n');
}

int main() {
    int data_count;
    scanf("%d", &data_count);
    for (int data = 1; data <= data_count; data++) Solve(data);
}