#include <iostream>
#include <string>
#include <set>
#include <algorithm>
#include <cstdio>
#include <vector>
#include <map>
#include <cstring>
#include <math.h>
using namespace std;
#define MAXA 1001
#define MAXB 1001
#define MAXH 1000000001
#define MAXW 405
#define INF 5000
#define MAXN 100001
#define MAXM 1001
#define MAXP 1000001
#define MOD 1000000007
#define EPS (1e-10)
typedef long long ll;
typedef pair<ll, ll> P;
ll N, A, B;

P max_element(ll n){
    ll ele = 1;
    ll lim = (ll)sqrt(n);
    for(ll i = 1;i <= lim;i++){
        if(!(n % i)){
            ele = i;
        }
    }
    P res = {ele, n / ele};
    return res;
}

int digitnum(ll n){
    int res = 1;
    while(n / 10){
        res++;
        n /= 10;
    }

    return res;
}


int main(){
    cin >> N;
    printf("%lf\n", N * M_PI);
    return 0;
}