postfix expression program

 #include<stdio.h>

int sp=-1,stack[20],i=0,op,res=0,r1;
char exp[20];
void main()
{
	clrscr();
	printf("\nEnter Postfix Expression :");
	gets(exp);
	for(i=0;exp[i] != '\0';i++)
	{
		if(exp[i] == '+' || exp[i] == '-' || exp[i] == '*' || exp[i] == '/')
		{
			flushall();
			op = stack[sp] ;
			sp--;
			r1 = stack[sp] ;
			switch(exp[i])
			{
				case '+' : res = op + r1;break;
				case '-' : res = op - r1;break;
				case '*' : res = op * r1;break;
				case '/' : res = op / r1;break;
			}
			stack[sp] = res;
		}
		else
		{
			sp++;
			stack[sp] = exp[i] - 48;
		}
	}
	printf("\n Result Is : %d ",res);
	getch();
}