Prog#138: copy last 'n' characters

/*
copy last 'n' characters 
Program#138
*/
#include<stdio.h>
main()
{
int j,i,n;
char a[50], b[50];
for(i=0;a[i]=getchar()!='$';i++);
a[i]='\0';
scanf("%d",&n);
j=0;
for(i=i-n;a[i]!='\0';i++)
{
b[i]=a[i];
j++;
}
b[j]='\0';
puts (b);
}