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
#include
#include
#include

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();
}