#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define ld long double
#define lll __int128
#define lp(i, a, b) for (int i = a; i < b; i++)
#define vec(v, type, sz) vector<type> v(sz)
#define in(v) \
    for (auto &i : v) cin >> i
#define all(v) v.begin(), v.end()
#define prv(v)                         \
    for (auto i : v) cout << i << " "; \
    cout << "\n"
#define MOD 1000000007
#define INF 1000000000
#define INFL 1000000000000000000ll
#define fast_io                       \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);                    \
    cout.tie(NULL);
#define vi vector<int>
#define vl vector<ll>
#define vii(v, n, m, val) vector<vector<int>> v(n, vector<int>(m, val))
#define viii(v, i, j, k, val)        \
    vector<vector<vector<int>>> v(i, \
                                  vector<vector<int>>(j, vector<int>(k, val)))
#define pii pair<int, int>
#define pb push_back
#define file_io                       \
    freopen("input.txt", "r", stdin); \
    freopen("output.txt", "w", stdout);

int main() {
	fast_io;
	double pi=atan(1)*4.0;
	double r; cin>>r;
	cout<<2*pi*r;
}