try yourself |run program

#include <stdio.h> 
#include <conio.h> 

void main(){
 int a;
 int b; 
 int c;  
 clrscr(); 
 printf("Enter value of a: "); 
 scanf("%d",&a); 
 printf("\n Enter value of b: "); 
 scanf("%d",&b);
 c = --a + --b; 
 printf("\n value of a = %d",a); 
 printf("\n value of b = %d",b); 
 printf("\n value of c = %d",c); 
 getch(); 
}

Program Output

Enter value of a: 10 
Enter value of b: 20 
value of a = 9 
value of b = 19 
value of c = 28