#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <limits>
#include <iomanip>
 
using namespace std;
 
typedef long long ll;
typedef long double ld;
#define L(ID) ((ID*2)+1)
#define R(ID) ((ID*2)+2)
#define M(L,R) ((L+R)/2)
#define forr(start,end) for(int i = start; i < end; i++)
#define gap ' '

const int MAXN = 200;
//ll a[MAXN], ST[MAXN * 4], LZ[MAXN*4];

/*bool perfectSquare(ll x){
    ll s = sqrt(x);
    if(s *s == x) return true;
    else return false;
}*/
typedef long double ld;

int main() { 
    ll r = 0;
    cin >> r;
    ld p = 3.14159;
    cout << setprecision(10) << r*2*p << endl;
  
    return 0; 
} 
