Program in C to reverse the first n characters in a file

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();

}