#define PI 3.14159
struct area
{
    float n;
    float area;
};
int main()
{
    struct area rad;
    scanf("%f",&rad.n);
    if(rad.n<0)
    {
        printf("Invalid input");
    }
    else
    {
        rad.area=3.14f*rad.n*
        printf("%.2f\n",area1);
    }
    return 0;
}