#include <stdio.h>
#include <stdlib.h>

int main()
{
    int k, n, m, i, j;

    scanf("%d%d%d", &k, &n, &m);

    if ((n != 1) && (m != 1))
        printf("-1");
    else {
        if ((m == 1) && (n == 1))
            printf("0");
        else if (m == 1) {
            printf("%d\n", n-1);
            for (i = 1; i < n; i++)
                printf("%d %d\n", i, i+1);
        }
        else {
            j = 0;
            for (i = m; i < k; i++)
                j += i;
            printf("%d\n", j);
            for (i = 1; i <= k-m; i++)
                for (j = i+1; j <= k; j++)
                    printf("%d %d\n", i, j);
        }

    }
    return 0;
}