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

int tohex(int c)
{
   if ((c >= '0') && (c <= '9')) {
        return c - '0';
   }
   if ((c >= 'a') && (c <= 'f')) {
        return c - 'a' + 10;
   }
   if ((c >= 'A') && (c <= 'F')) {
        return c - 'A' + 10;
   }
   return 0;
}

int isspace(int c)
{
   if (c <= ' ') return 1;
   if (c == '\n') return 1;
   if (c == '\t') return 1;
   if (c == ':') return 1;
   if (c == ';') return 1;
   if (c == ',') return 1;
   return 0;
}

void verify_nibble(int nibble, int current)
{
   if (nibble != 0) {
        fprintf(stderr,"count mismatch %d (nibbles=0x%x)\n",nibble,current);
        fflush(stderr);
   }
}

int main(int argc, char **argv)
{
   int c;
   int current = 0;
   int nibble = 0;
   int skip = 0;

   if (argv[1]) {
        skip = atoi(argv[1]);
   }

#define NIBBLE_COUNT 2
   while ((c=getchar()) != EOF) {
      if (isspace(c)) {
        verify_nibble(nibble,current);
        continue;
      }
      if (skip) {
        skip--;
        continue;
      }
      current = current << 4 | tohex(c);
      nibble++;
      if (nibble == NIBBLE_COUNT) {
        putchar(current);
        nibble = 0;
        current = 0;
      }
   }
   return 0;
}
