#include <iostream>

using namespace std;
long long n, k, m, nr;
int main()
{
    int i, j;
    cin>>k>>n>>m;
    if( (m!=1 && n!=1) || (m>k || n>k) )
    {
        cout<<"Impossible";
        return 0;
    }
    k=k-m+1;

    cout<<(k-1)*k/2<<'\n';
   
   	for(i=1; i<k; ++i)
    {
        for(j=i+1; j<=k; ++j)
        	cout<<i<<' '<<j<<'\n';
    }
    return 0;
}