#include<stdio.h>
#include<math.h>
struct circle{
    float radius;
    float area; 
};
int main(){
    struct circle c;
    scanf("%f",&c.radius);
    if(c.radius<0)
    {
        printf("Invalid input")
    }
    else
    {
        c.area=3.141592653589793*c.radius*c.radius;
        printf("%.2f/n",c.area);
    }
    return0;
}