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

extern const char *__progname;

static int allhex(const char *s)
{
 while (1)
  { switch (*s++)
     { case '0': case '1': case '2': case '3':
       case '4': case '5': case '6': case '7':
       case '8': case '9': case 'a': case 'b':
       case 'c': case 'd': case 'e': case 'f':
	  break;
       case '\0':
	  return(1);
	  break;
       default:
	  return(0);
	  break;
     }
  }
}

int main(int, char **);
int main(int ac, char **av)
{
 int i;

 if ( (ac != 3) ||
      (strlen(av[1]) != strlen(av[2])) ||
      !allhex(av[1]) ||
      !allhex(av[2]) )
  { fprintf(stderr,"Usage: %s blk1 blk2\n",__progname);
    exit(1);
  }
 for (i=0;av[1][i];i++)
  { int a;
    int b;
    sscanf(av[1]+i,"%1x",&a);
    sscanf(av[2]+i,"%1x",&b);
    printf("%x",a^b);
  }
 printf("\n");
 return(0);
}