/**
 * Author RDP
 * 
**/
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <cctype>
#include <string>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <queue>
#include <stack>
#include <algorithm>
#include <functional>
#include <bitset>
#include <iomanip>
#define endl '\n'
#define bp(x) __builtin_popcount(x)
#define zf(x) __builtin_clz(x)
#define zl(x) __builtin_ctz(x)
#define par(x) __bultin_parity(x)
#define FAST_IO ios::sync_with_stdio(0); std::cin.tie(0);
typedef long long ll;
using namespace std;

void test_case()
{
    FAST_IO
    double r;
    cin >> r;
    cout <<6.2831 * r <<endl;
    return ;
}
int main()
{
    FAST_IO
    test_case();
    return 0;
}