实验3

#include<stdio.h>
int main()
{
    double r,s;


    printf("Enter r:");
    scanf("%Lf",&r);
    
    if(r<=0)
    {
       printf("sorry!it is wrong!');
    }
    else 
    {
        s=3.1415923*r*r;
        printf("s=%.2f
",s);
    }
    
    return 0;
}