#include<stdio.h>
#include<math.h>
#define MOD 998244353
#define MAX 3001
#define MAX_DISTANCE 501
#define PRECISION 0.00000000001
#define INF 4000000000000000000
int main()
{
    double cir;
    scanf("%lf",&cir);
    cir=cir*2*acos(-1);
    printf("%.8f",cir);
    return 0;
}
