//----------------------------------------------------------------------
#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)   FOR(i,0,n)
//----------------------------------------------------------------------

using namespace std;
using ll = long long;
using P = pair<int,int>;
using Graph = vector<vector<int>>;

int main(void) {
    double r;
    cin >> r;
    cout << setprecision(10);
    cout << 2 * r *  M_PI << endl;
    return 0;
}