C program to swap two numbers without using third variable

1.  #include<stdio.h>
2. int main()
3. {
4.   int a = 10;
5.   int b = 20;
6.   printf("Before swap a=%d b=%d\n",a,b);
7.     a = a+b;
8.     b = a-b;
9.     a = a-b;
10.   printf("\nAfter swap a=%d b=%d\n",a,b);
11.   return 0;
12. }