/*
vector< vector<int> > Matrix(10, vector<int>(20, -1));
iterator max_element(range);
iterator min_element(range);
sort ALLR descending
string s
istringstream is(s); is >> x;
ostringstream os(s); os << x;

for (auto x : container)
{
cout << x << endl;
}

for_each(v.begin(), v.end(),
[&] (int x) {
cout << x << endl;
}
)


*/

/*
------------------------------------------------------------------
void dfs(int i) {
if (!V[i]) {
V[i] = true;
for_each(all(W[i]), dfs);
}
}

bool check_graph_connected_dfs() {
int start_vertex = 0;
V = vi(N, false);
dfs(start_vertex);
return (find(all(V), 0) == V.end());
}

void add(int poz, int val) {
for (; poz <= N; poz += zeroes(poz)) {
aib[poz] += val;
}
}

long long compute(int poz) {
long long sum = 0;
for (; poz; poz -= zeroes(poz)) {
sum += aib[poz];
}
return sum;
}
*/

#include <fstream>
#include <iostream>
#include <string>
#include <complex>
#include <cmath>
#include <set>
#include <vector>
#include <map>
#include <queue>
#include <cstdio>
#include <stack>
#include <algorithm>
#include <list>
#include <ctime>
#include <cassert>
#include <iomanip>

using namespace std;

//```
#define ONLINE_JUDGE1

const double epsilon = 1e-7;
#define LL long long
#define ULL unsigned long long 

typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef vector<double> vd;
typedef vector<string> vs;
typedef pair<int, int> ii;
typedef pair<long long, long long> ll;
typedef vector<ii> vii;
typedef vector<ll> vll;

#define all(V) V.begin(), V.end()
#define allr(V) V.rbegin(), V.rend()
#define for_c_it(container, it) for (auto it = container.begin(); it != container.end(); it++)
#define present(container, element) (container.find(element) != container.end()) 
#define sz(a) int((a).size()) 
#define pb push_back 
#define mp make_pair
#define zeroes(x) ((x ^ (x - 1)) & x)

int K, N, M;

int main() {
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
#endif

	int N;
	cin >> K >> N >> M;
	if (N > K || M > K) {
		cout << -1;
		return 0;
	}
	if (K == 1 && N == 1 && M == 1) {
		printf("0");
	}
	else if (K == 2 && N == 2 && M == 1) {
		printf("1\n1 2");
	}
	else if (K == 2 && N == 1  && M == 2) {
		printf("0");
	}
	else if (K == 3 && N == 1) {
		printf("0");
	}
	else if (K == 3 && N == 2) {
		printf("1\n1 2");
	}
	else if (K == 3 && N == 3 && M < 3) {
		printf("2\n1 2\n2 3");
	}
	else if (K > 3) {
		printf("%d", K - 1);
		for (int i = 1; i < K; ++i) {
			printf("\n%d %d", i, i + 1);
		}
	}
	else printf("-1");

#ifndef ONLINE_JUDGE
	while (true);
#endif
	return 0;
}