codeforces 639B Bear and Forgotten Tree 3

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int n,d,h,p=1,ret=1;
int main()
{
    scanf("%d%d%d",&n,&d,&h);
    if (2*h<d) {printf("-1\n");return 0;}
    if (d==1) 
    {
        if (n==2) printf("1 2\n");
        else printf("-1\n");
        return 0;
    }
    for (int i=1;i<=h;i++)
        printf("%d %d\n",i,i+1);
    p=h+2;
    for (int i=1;i<=d-h;i++)
    {
        printf("%d %d\n",ret,p);
        ret=p;p++;
    }
    if (p!=n+1)
    {
        if (d!=h)
        {
            for (int i=p;i<=n;i++)
                printf("1 %d\n",i);
        }
        else
        {
            for (int i=p;i<=n;i++)
                printf("2 %d\n",i);
        }
    }
    return 0;
}