#include <iostream>
#include <stdio.h>
#include <vector>
#include <algorithm>
#include <cmath>
#define rep(i,cc,n) for(int i=cc;i<=n;++i)
#define drep(i,cc,n) for(int i=cc;i>=n;--i)
typedef long long ll;
using namespace std;

int main(){
    const double PI=3.14159265358979323846;
    int r;
    cin>>r;
    double ans = 0;
    ans = PI*(double)r*2.0;
    printf("%.14f\n",ans);
    return 0;
}