Reverse First Letter characters

/* Write a C program to reverse the first n characters in a file.

(Note: The file name and n are specified on the command line.)*/

#include <stdio.h>

#include <conio.h>

#include <string.h>

#include <process.h>

void main(int argc, char *argv[])

{

  char a[15];

  char s[20];

  char n;

  int k;

  int j=0;

  int i;

  int len;

  FILE *fp;

  if(argc!=3)

  {

  puts("Improper number of arguments.");

  exit(0);

  }

  fp = fopen(argv[1],"r");

  if(fp == NULL)

  {

  puts("File cannot be opened.");

  exit(0);

  }

  k=*argv[2]-48;

  n = fread(a,1,k,fp);

  a[n]='\0';

  len=strlen(a);

  for(i=len-1;i>=0;i--)

  {

   s[j]=a[i];

   printf("%c",s[j]);

   j=j+1;

}

s[j+1]='\0';

getch();

}