/* Include lib */
#include <stdio.h>
#include <assert.h>

/* Define */

/* API calc power of 2 */
/* Calc 2^k with k is natural number (positive integer, except 0) */
int power_of_2(int k, int *ret)
{
  int i, tmp = 1;

  if (k <= 0)
    return -1;
  
  for (i = 0; i < k; i++)
    tmp *= 2;
  
  *ret = tmp;
  return 0;
}

/* Main func interract with user */
int main(void)
{
  int sel, k, ret;

  while (1) {
    printf("\n\nSelection:\n");
    printf("0: input k for calculating 2^k\n");
    printf("1: Exit\n");
    printf("Select: ");
    scanf("%d", &sel);

    if (sel == 0) {
      printf("Input k: ");
      scanf("%d", &k);
      if (!power_of_2(k, &ret)) {
        printf("=> %d\n", ret);
      } else {
        printf("Input must be natural (integer > 0)\n");
      }
    } else {
      return 0;
    }
  } 

  return 0;
}