#include<bits/stdc++.h>
#define endl '\n'
#define int long long
#define rep(i, a, b) for(int i = a; i <= b; i ++ )
#define pre(i, a, b) for(int i = a; i >= b; i -- )
using namespace std;
const int N = 1e6 + 10;
int n, m, k;
int cnt[N];
int l[N], r[N], f[N], son[N];
void solve()
{
    cin >> n >> m;
    rep(i, 1, n) cnt[i] = 0, f[i] = 0;
    rep(i, 1, m)
    {
        cin >> l[i] >> r[i];
    }
    int w = 0, rt = 0, tt = 0;
    rep(i, 1, m)
    {
        if(cnt[l[i]] || cnt[r[i]])
        {
            cout << "IMPOSSIBLE" << endl;
            return;
        }
        cnt[l[i]] = 1;
        cnt[r[i]] = 1;
        if(l[i] == r[i]) w ++ ;
    }
    if(w == n)
    {
        cout << "IMPOSSIBLE" << endl;
        return;
    }
    int ma = 0;
    rep(i, 1, m)
    {
        if(r[i] - l[i] + 1 > ma) ma = r[i] - l[i] + 1, rt = l[i], tt = r[i];
    }
    rep(i, rt + 1, tt) f[i] = i - 1;
    rep(i, 1, m)
    {
        if(l[i] == rt) continue;
        f[l[i]] = rt;
        rep(j, l[i] + 1, r[i]) f[j] = j - 1;
    }
    rep(i, 1, n) cout << f[i] << " ";
    cout << endl;
}
signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int _;
    _ = 1;
    cin >> _;
    while(_ -- )
    {
        solve();
    }
    return 0;
}