#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define all(v) v.begin(),v.end()
#define mem(arr) memset(arr, 0, sizeof(arr))
#define ff first
#define ss second
#define int long long int
const int MOD = 1e9 + 7;
const int INF = 1e18;
const int N = 1e6 + 20;
vector<int> g[N];
vector<int> vis(N);
int32_t main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    int tc = 1;
    while(tc--){
        const double pi = 3.14;
        //cout << pi << '\n';
        double r;
        cin >> r; 
        cout << 2 * pi * r << '\n';  
    }
}