text: #include <stdio.h>
  
  long
  factorial(long n)
  {
  	return (n == 0 || n == 1) ? 1 : n * factorial(n-1);
  }
  
  long
  combinations(long m, long n) // m <= n
  {
  	return factorial(n)/(factorial(m)*factorial(n-m));
  }
  
  int
  main(void)
  {
  	printf("%ld\n", combinations(1, 20));
  	return 0;
  }
  