//Code by Patcas Csaba

#include <vector>
#include <string> 
#include <set> 
#include <map> 
#include <queue> 
#include <bitset> 
#include <stack>
#include <list>

#include <numeric> 
#include <algorithm> 

#include <cstdio>
#include <fstream>
#include <iostream> 
#include <sstream> 
#include <iomanip>

#include <cctype>
#include <cmath> 
#include <ctime>
#include <cassert>

using namespace std;

#define LL long long
#define PII pair <int, int>
#define VB vector <bool>
#define VI vector <int>
#define VD vector <double>
#define VS vector <string>
#define VPII vector <pair <int, int> >
#define VVI vector < VI >
#define VVB vector < VB >

#define FORN(i, n) for(int i = 0; i < (n); ++i)
#define FOR(i, a, b) for(int i = (a); i <= (b); ++i)
#define FORD(i, a, b) for(int i = (a); i >= (b); --i)
#define REPEAT do{ 
#define UNTIL(x) }while(!(x)); 

#define SZ size()
#define BG begin() 
#define EN end() 
#define CL clear()
#define X first
#define Y second
#define RS resize
#define PB push_back
#define MP make_pair
#define ALL(x) x.begin(), x.end()

#define IN_FILE "a.in"
#define OUT_FILE "a.out"

int k, m, n, nrEdges = 0;
VVI g;

int main()
{
	//Read data
	//freopen(IN_FILE, "r", stdin);
	//freopen(OUT_FILE, "w", stdout);
	
	cin >> k >> m >> n;
	if (n != 1 && m != 1)
	{
		cout << "Impossible";
		return 0;
	}

	if (n == 1 && m == 1)
	{
		if (k == 1) cout << "0";
		else 
			if (k == 2 || k == 3) cout << "Impossible";
			else
			{
				cout << k - 1 << endl;
				FOR(i, 1, k - 1) cout << i << " " << i + 1 << endl;
			}
		return 0;
	}

	//Solve
	g.RS(k + 1, VI(k + 1));
	if (m == 1)
	{
		if (n > k)
		{
			cout << "Impossible";
			return 0;
		}
		FOR(i, 1, k - n)
			FOR(j, i + 1, k - n + 1)
			{
				++nrEdges;
				g[i][j] = 1;
				g[j][i] = 1;
			}
	}
	else
	{
		if (m > k)
		{
			cout << "Impossible";
			return 0;
		}
		FOR(i, 1, k - m)
			FOR(j, i + 1, k - m + 1)
			{
				g[i][j] = 1;
				g[j][i] = 1;
			}
		FOR(i, 1, k - 1)
			FOR(j, i + 1, k)
			{
				g[i][j] = 1 - g[i][j];
				g[j][i] = 1 - g[j][i];
				if (g[i][j] == 1) ++nrEdges;
			}
	}


	//Write data
	cout << nrEdges << endl;
	FOR(i, 1, k - 1)
			FOR(j, i + 1, k)
				if (g[i][j]) cout << i << " " << j << endl;
	return 0;
}