#include <_timeval.h>
#include <chrono>
#include <iostream>
#include <ostream>
#include <vector>
int main() {
std::ios::sync_with_stdio(0);
std::cin.tie(nullptr);
std::cout.tie(nullptr);
int T;
std::cin >> T;
for (int TC = 1; TC <= T; TC++) {
std::cout << "Case #" << TC << ": ";
int n, m;
std::cin >> n >> m;
std::vector<std::vector<int>> ed(n), rved(n);
for (int i = 0; i < m; i++) {
int u, v;
std::cin >> u >> v;
ed[--u].push_back(--v);
rved[v].push_back(u);
}
std::vector<bool> con1(n), con2(n);
int lst;
auto scc1 = [&](auto scc1, int u) -> void {
con1[u] = 1;
for (auto v : ed[u])
if (!con1[v])
scc1(scc1, v);
lst = u;
};
bool imp = 0;
scc1(scc1, 0);
for (int i = 0; i < n; i++) {
if (!con1[i]) {
imp = 1;
break;
}
}
auto scc2 = [&](auto scc2, int u) -> void {
con2[u] = 1;
for (auto v : rved[u])
if (!con2[v])
scc2(scc2, v);
};
scc2(scc2, lst);
for (int i = 0; i < n; i++) {
if (!con2[i]) {
imp = 1;
break;
}
}
if (imp) {
std::cout << "IMPOSSIBLE\n";
continue;
}
std::vector<int> lnk(n, -1), vis(n, 0);
auto dfs = [&](const auto &dfs, const int &u, std::vector<bool> &vis) -> bool {
for (int v : ed[u]) {
if (vis[v])
continue;
vis[v] = 1;
if (!~lnk[v] || dfs(dfs, lnk[v], vis)) {
lnk[v] = u;
return true;
}
}
return false;
};
int cnt = 0;
for (int i = 0; i < n; i++) {
std::vector<bool> vis(n, 0);
if (dfs(dfs, i, vis))
cnt++;
}
if (cnt < n) {
std::cout << "IMPOSSIBLE\n";
continue;
}
std::vector<std::vector<int>> nwed(n);
for (int u = 0; u < n; u++) {
if (imp)
break;
for (auto v : ed[u]) {
if (lnk[v] == u) {
for (int i = 0; i < n; i++) {
nwed[lnk[i]].push_back(i);
}
continue;
}
for (int i = 0; i < n; i++)
if (lnk[i] == u)
lnk[i] = -1;
int tmp = lnk[v];
lnk[v] = u;
std::vector<bool> vis(n, 0);
vis[v] = 1;
if (!dfs(dfs, tmp, vis)) {
imp = 1;
break;
}
for (int i = 0; i < n; i++) {
nwed[lnk[i]].push_back(i);
}
}
}
if (imp) {
std::cout << "IMPOSSIBLE\n";
continue;
}
std::cout << n * m + 1 << "\n";
std::vector<int> ans(n * m);
int cans = 0;
auto fnd = [&](const auto &fnd, const int &u) -> void {
while (nwed[u].size()) {
int v = nwed[u].back();
nwed[u].pop_back();
fnd(fnd, v);
ans[cans++] = u;
}
};
fnd(fnd, 0);
int st = 0;
for (; ans[st] != 0; st++)
;
for (int i = st; ~i; i--) {
std::cout << ans[i] + 1 << " ";
}
for (int i = n * m - 1; i > st; i--) {
std::cout << ans[i] + 1 << " ";
}
std::cout << "1\n";
}
}