ISC 2013 Practical - Expected Question

Sum of two Binary numbers

import java.util.*;
class BinarySum
{
public int n,m,p,q,r;
public void input()
{
// two binary numbers added by converting into decimal
// and sum then converted into binary
Scanner br = new Scanner(System.in);
System.out.println(" Input a binary number n");
n=br.nextInt();
System.out.println(" Input a binary number m");
m=br.nextInt();
}
public void calculate()
{
p=bintodec(n);
q=bintodec(m);
if(p== -1 || q== -1)
System.out.println("INVALID BINARY VALUE ");
else
{
System.out.println("Binary number " + n + " in decimal is " + p);
System.out.println("Binary number " + m + " in decimal is " + q);
r=sumbin(p+q);
System.out.println("Sum of two binary number " + n + " and " + m + " is " + r);
}
}
public int bintodec(int x)
{
int a,s=0, i=0;
while(x>0)
{
a=x%10;
if(a>1) return -1;
s = s + (int)Math.pow(2,i)*a;
x=x/10;
i++;
}
return s;
}
public int sumbin(int x)
{
int a,s=0, i=0;
while(x>0)
{
a=x%2;
s = s + (int)Math.pow(10,i)*a;
x=x/2;
i++;
}
return s;
}
public static void main(String arg[])
{
BinarySum ob= new BinarySum();
ob.input();
ob.calculate();
}
}