Subtraction Using Twos Complement method

import java.util.Scanner;


public class Subtraction {
    int flag = 0;

    StringBuffer add(StringBuffer s1,StringBuffer s2) {
         StringBuffer s3=new StringBuffer();
         System.out.println("N1 IN BINARY IS :  "+s1+"\nN2 IN BINARY IS : "+s2);
         s1=s1.reverse();
         s2=s2.reverse();
         int len=s1.length()>s2.length()?s1.length():s2.length();
         while(s1.length()<(len)) {
             s1.append("0");
         }
         while(s2.length()<(len)) {
             s2.append("0");
         }
     for (int i = 0; i < s1.length(); i++) {
             if (flag == 0) {
                     if (s1.charAt(i) == '0' && s2.charAt(i) == '0')
                             s3.append("0");
                     if (s1.charAt(i) == '1' && s2.charAt(i) == '0')
                             s3.append("1");
                     if (s1.charAt(i) == '0' && s2.charAt(i) == '1')
                             s3.append("1");
                     if (s1.charAt(i) == '1' && s2.charAt(i) == '1') {
                             flag = 1;
                             s3.append("0");
                     }

             } else {
                     if (s1.charAt(i) == '0' && s2.charAt(i) == '0') {
                             s3.append("1");
                             flag = 0;
                     }
                     if (s1.charAt(i) == '1' && s2.charAt(i) == '0') {
                             s3.append("0");
                             flag = 1;
                     }
                     if (s1.charAt(i) == '0' && s2.charAt(i) == '1') {
                             s3.append("0");
                             flag = 1;
                     }
                     if (s1.charAt(i) == '1' && s2.charAt(i) == '1') {
                             flag = 1;
                             s3.append("1");
                     }
             }
     }


     return s3;
    }
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        Subtraction sub=new Subtraction();
        System.out.println("Enter the two numbers : ");
        int a=sc.nextInt();
        int b=sc.nextInt();
        int bothNegative=0;
        if(a<0&&b<0)
            bothNegative=1;
        StringBuffer s1=new StringBuffer(Integer.toBinaryString(a));
        StringBuffer s2=new StringBuffer(Integer.toBinaryString(b));
        StringBuffer s3=sub.add(s1, s2);
        String s4=new String (s3.reverse());
        long l = Long.parseLong(s4, 2);
        int i=(int)l;
        System.out.println("THE ANSWER IS : "+i);
    }

}