#include<stdio.h>

int l3_read(char *s,int max) {
  int len;
  char bit=0;
  int a;
  len=l2_read(s,max);
  //compute checksum
  for(a=0;a<len-1;a++) {
    bit=s[a]+bit;
  }
  if(bit!=s[a]) {
    //checksum error
    printf("Checksum error\n");
    return -1;
  }
  return(len-1);
}

int l3_write(char *s,int n) {
  char *buff;
  char bit=0;
  int a,retval;

  //allocate space
  if(!(buff=(char *)malloc(n+1))) {
    printf("Malloc error\n");
    return -1;
  }

  //compute checksum
  for(a=0;a<n;a++) {
    bit=s[a]+bit;
    buff[a]=s[a];
  }
  buff[a]=bit;
  retval=l2_write(buff,n+1);

  //free memory!!!!
  free(buff);
  if(retval==-1) {
    return -1;
  }
  return n;
}