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 = 30