圆面积

#include<stdio.h>
int main()
{
    double r,s;
    printf("enter r:
");
    scanf("%lf",&r);

    if(r<=0)
        printf("输入错误，重新输入");
    
    else
        s=3.14*r*r;
        printf("s=%lf
",s);

    return 0;
}