#include <iostream>
#include <algorithm>
#include <vector>
#include <utility>
#include <set>
using std::cin, std::cout;

#define F(i, a, b) for(int i = a; i <= (b); ++i)
#define F2(i, a, b) for(int i = a; i < (b); ++i)
#define dF(i, a, b) for(int i = a; i >= (b); --i)

void Solve();
int main() {
    cin.sync_with_stdio(false);
    cin.tie(nullptr);
    int tests = 1;
    cin >> tests;
    while (tests--)
        Solve();
    return 0;
}

using LL = long long;
const int Mod = 998244353;

const int Inf = 0x3f3f3f3f;
const LL InfLL = 0x3f3f3f3f3f3f3f3f;

const int MN = 500005;

int n, k;
std::vector<std::pair<int, int>> V[MN];
int p[MN];

void Solve() {
    cin >> n >> k;
    F(x, 1, n)
        V[x].clear();
    int mx = 0;
    F(i, 1, k) {
        int l, r;
        cin >> l >> r;
        V[r - l + 1].push_back({l, r});
        mx = std::max(mx, r - l + 1);
    }
    if ((int)V[mx].size() >= 2) {
        int px = 0;
        dF(x, mx - 1, 1)
            if (V[x].empty()) {
                px = x;
                break;
            }
        if (!px)
            return void(cout << "IMPOSSIBLE\n");
        bool ok = false;
        dF(x, px - 1, 1)
            if (!V[x].empty())
                ok = true;
        if (!ok)
            return void(cout << "IMPOSSIBLE\n");
    }
    F(x, 1, n)
        for (auto [l, r] : V[x])
            F(i, l + 1, r)
                p[i] = i - 1;
    int pp = 0;
    while (true) {
        if ((int)V[mx].size() == 1) {
            int rt = V[mx][0].first;
            p[rt] = pp;
            F(x, 1, mx - 1)
                for (auto [l, r] : V[x])
                    p[l] = rt;
            break;
        }
        auto [l0, r0] = V[mx].back();
        p[l0] = pp;
        F(j, 0, (int)V[mx].size() - 2)
            p[V[mx][j].first] = l0;
        V[mx - 1].push_back({l0 + 1, r0});
        pp = l0;
        --mx;
    }
    F(i, 1, n)
        cout << p[i] << " \n"[i == n];
}