/* Simple trial division microbenchmark. */
#include <stdio.h>
#include <stdlib.h>

int 
main(int argc, char **argv)
{
  int max;
  if (argc != 2 || (max = atoi(argv[1])) == 0) {
    fprintf(stderr, "Usage: %s 53\n"
            "Prints the last prime number before 53\n",
            argv[0]);
    return -1;
  }

  if (max <= 2) return 1;
  if (max == 3) {
    printf("2\n");
    return 0;
  }

  for (int p = max % 2 ? max - 2 : max - 1; p > 0; p -= 2) {
    for (int i = 3; i * i <= p; i += 2) {
      if (p % i == 0) goto composite;
    }
    printf("%d\n", p);
    return 0;
  composite:;
  }

  return -1;
}