#include <bits/stdc++.h>
#define int long long
#define lson k<<1
#define rson (k<<1)|1
#define debug cout<<666<<endl;
using namespace std;
const int N=1e6+5;
int l[N],r[N],a[N];
bool cmp(int a,int b)
{
    return r[a]-l[a]+1>r[b]-l[b]+1;
}
int n,k;
int id;
int fa[N];
int ne[N];
int dfs(int u,int dep)
{
    if(ne[u]==0)
    {
        return -1;
    }
    if(dep>r[a[id]]-l[a[id]]+1)
    {
        for(int i=id;i<=k;i++)
        {
            int la=u;
            for(int j=l[a[i]];j<=r[a[i]];j++)
            {
                fa[j]=la;
                la=j;
            }
        }
        return 1;
    }
    while(id<=k&&dep==r[a[id]]-l[a[id]]+1)
    {
        int la=u;
        for(int i=l[a[id]];i<=r[a[id]];i++)
        {
            fa[i]=la;
            la=i;
        }
        id++;
    }
    return dfs(ne[u],dep-1);
}
void vision()
{
    cin>>n>>k;
    int ma=0;
    int cnt=0;
    for(int i=1;i<=k;i++) 
    {
        cin>>l[i]>>r[i];
        a[i]=i;
    }
    sort(a+1,a+1+k,cmp);
    for(int i=l[a[1]];i<=r[a[1]];i++)
    {
        ne[i]=i+1;
        fa[i]=i-1;
    }
    fa[l[a[1]]]=0;
    ne[r[a[1]]]=0;
    id=2;
    // cout<<a[1]<<"\n";
    if(dfs(l[a[1]],r[a[1]]-l[a[1]]+1)==-1) cout<<"IMPOSSIBLE\n";
    else 
    {
        for(int i=1;i<=n;i++) cout<<fa[i]<<" ",fa[i]=0,ne[i]=0;
        cout<<"\n";
    }
    return ;
}
signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int t=1;
    cin>>t;
    while(t--){
        vision();
    }
    return 0;
}