/* Program to compute the number of distinct sequences of length m that can
   be formed from 2 different letters, with n1 available copies of the first
   letter and n2 copies of the second.

   Put N=n1+n2 and assume that 0<m=<N.

   Author: M.V.Wickerhauser
   Date: 30 Jan 2005

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

static int count;		/* Accumulate the answer here */

void usage(char *name) 
{
  fprintf(stderr, "Usage: %s  m\n where\n\t m=sequence length.\n", name);
  exit(1);
}

void
deduct2(int m, int n1, int n2)
{
  if(m==0) {			/* last letter chosen; count the sequence */
    ++count;
  } else {			/* ...else choose another available letter */
    if(n1>0) deduct2 ( m-1, n1-1, n2 );
    if(n2>0) deduct2 ( m-1, n1, n2-1 );
  }
  return;
}

int
main(int argc, char **argv) {

  int m, n1, n2;
  n1=3;				/* Number of first letters available */
  n2=2;				/* Number of second letters available */
  if(argc==2) m=atoi(argv[1]);	/* Length is read from command line */
  else usage(argv[0]);		/* ...crash on bad command line */

  deduct2(m,n1, n2);		/* Recursive counting */
  printf("%d\n", count);	/* Output */
  return 0;
}