#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define pll pair < ll , ll >
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define ld long double
#define endl "\n"
using namespace std;
using namespace __gnu_pbds;
ll M = 1e9 + 7;

typedef tree<
int,
null_type,
less<int>,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;

int main()
{

    ios_base :: sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int p;
    cin >> p;
    cout << fixed << setprecision(12) << p*3.141592653589793238462*2;
}
