实验1

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

	printf("r");
	scanf("%lf",&r);

    if(r<=0)
		printf("Error!");
	else if(r>0){
			s=3.1415926*r*r;
		}
	printf("f=%.7f\n",s);

	return 0;
}