#include<bits/stdc++.h>
#define in cin
#define out cout
#define abs(x) ((x>0)?(x):(-(x)))
#define max(a,b) ((a)>(b)?(a):(b))
#define min(a,b) ((a)<(b)?(a):(b))
#define FOR(i, a, b) for(int i = a; i <= b; ++i)
#define DOWNFOR(i, a, b) for(int i = a; i >= b; --i)
#define FOREACH(i, t) for (typeof(t.begin()) i = t.begin(); i != t.end(); ++i)
using namespace std;
typedef long long ll;
int N,a,b,K,m[101][101];
int x[10001],y[10001];
int main(){
    #ifndef ONLINE_JUDGE
    ifstream in("test.in");
    ofstream out("test.out");
    #endif
    in>>N>>a>>b;
    if(min(a,b)>1){
        out<<"Impossible\n";
        return 0;
    }
    if(a==1 && b==1){
        if(N==2 || N==3){
            out<<"Impossible\n";
            return 0;
        }
        out<<N-1<<'\n';
        for(int i=1;i<N;i++) out<<i<<' '<<i+1<<'\n';
        return 0;
    }
    int P;
    if(a>1) P=N-a+1;
    else P=N-b+1;
    for(int i=1;i<=P;i++) for(int j=i+1;j<=P;j++) m[i][j]=1;
    if(b>1){ for(int i=1;i<=N;i++) for(int j=i+1;j<=N;j++) if(m[i][j]) x[++K]=i,y[K]=j; }
    else{ for(int i=1;i<=N;i++) for(int j=i+1;j<=N;j++) if(!m[i][j]) x[++K]=i,y[K]=j; }
    out<<K<<'\n';
    for(int i=1;i<=K;i++) out<<x[i]<<' '<<y[i]<<'\n';
    return 0;
}