#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define ll long long
#define BIT_FLAG_0 (1<<0) // 0000 0000 0000 0001
#define BIT_FLAG_1 (1<<1) // 0000 0000 0000 0010
#define BIT_FLAG_2 (1<<2) // 0000 0000 0000 0100
#define BIT_FLAG_3 (1<<3) // 0000 0000 0000 1000
#define BIT_FLAG_4 (1<<4) // 0000 0000 0001 0000
#define BIT_FLAG_5 (1<<5) // 0000 0000 0010 0000
#define BIT_FLAG_6 (1<<6) // 0000 0000 0100 0000
#define BIT_FLAG_7 (1<<7) // 0000 0000 1000 0000

int main(){
    int r;
    long double ans;
    cin >> r;
    ans=r*2*M_PI;
    cout << fixed << setprecision(20) << ans << endl;
    return 0;
}
#include <iostream>
using namespace std;
int main()
{
 double U = 3.14;
 int R;
cin >> R  >> U; 
 cout <<  R * 2 * U <<"\n";
}