#include<stdio.h>
#include<math.h>
int main()
{
    float f;
    while(scanf("%f",&f) != EOF)
    {
        int sign=1;
        double pai=1.0,n=1.0,term=1.0;
        while(fabs(term)>=f)
        {
            n=n+2;
            sign=-sign;
            term=sign/n;
            pai=pai+term;
            
        }
        pai=pai*4;
        printf("%.6lf\n",pai);
    }
    return 0;
}