#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(long long i = 0;i < n;i++)
#define repr(i, n) for(int i = n;i >= 0;i--)
#define For(i, s, e) for(int i = s;i <= e;i++)
#define Sort(v, n) sort(v, v+n);
#define VSort(v) sort(v.begin(), v.end());
#define ll long long
#define pb(a) push_back(a)
#define INF 999999999
#define cY cout<<"Yes";
#define cN cout<<"No";
#define cA(a) cout<<a;
const ll MOD = 1000000007;


int main() {
    int r;cin>>r;
    cout << r*6.283;
}