2. 输入一个圆半径r，当r＞＝0时，计算并输出圆的面积和周长，否则，输出提示信息。

#include <stdio.h>
   #define PI 3.14 
int main(void)
{ double  r ,area , girth;  
 printf("please input r:\n");
 scanf("%lf",&r);
	 if (r>=0)                        
	      { area =PI*r*r;
           girth =2*PI*r ;
	        printf("the area is %.2f\n", area);	
printf("the girth is %.2f\n", girth);}
else  
printf("Input error!\n"); 
return 0;
 }