C一个简单的加减程序

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
int main()
{
float num,num2;
char sss,a;
printf("说明\n");
do
{
clrscr();
a='n';
scanf("%f",&num);
sss=getchar();sss=getch();
if(sss!='+'||sss!='-'||sss!='/'||sss!='*')
{printf("错误！\n");system("pause");}
scanf("%f",&num2);a=getchar();
if(sss=='+') printf("%.2f\n",num+num2);
if(sss=='-') printf("%.2f\n",num-num2);
if(sss=='*') printf("%.2f\n",num*num2);
if(sss=='/') printf("%.2f\n",num/num2);
printf("按Y结束此程序或按任意键继续\n");
a=getch();
}while(a!='y'||a!='Y');