#include <bits/stdc++.h>
using namespace std;

#define int long long

const int N = 1e6 + 5;

int n, m;
map<int, int> cnt;
vector<array<int, 2>> all;

void solve()
{
    cin >> n >> m;
    cnt.clear(), all.clear();

    int ma = 0;
    for (int i = 1; i <= m; i++) {
        int l, r;
        cin >> l >> r;
        cnt[r - l + 1]++;
        ma = max(ma, r - l + 1);
        all.push_back({ l, r });
    }
    if (cnt[ma] != 1) {
        cout << "IMPOSSIBLE\n";
        return;
    }
    sort(all.begin(), all.end(), [](array<int, 2>& p1, array<int, 2>& p2) {
        return p1[1] - p1[0] > p2[1] - p2[0];
    });
    vector<int> ans(n + 1, 0);
    for (int i = 0; i < m; i++) {
        int l = all[i][0], r = all[i][1];
        if (i)
            ans[l] = all[0][0];
        for (int j = l + 1; j <= r; j++) {
            ans[j] = j - 1;
        }
    }
    for (int i = 1; i <= n; i++) {
        cout << ans[i] << ' ';
    }
    cout << '\n';
}

signed main()
{
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);

    int T = 1;
    cin >> T;
    while (T--) {
        solve();
    }
}