#include<bits/stdc++.h>
#define ll long long int
const ll MOD = 1e9 + 7;
using namespace std;
ll powmod(ll a, ll b, ll MOD = MOD){
    if(!a and !b)	return 1ll;
    if(!a)	return a;
    ll res = 1ll;
    b %= (MOD - 1ll), a %= MOD;
    while(b){
        if(b%2 == 1)	res = (res * a) % MOD;
        a = (a * a) % MOD;
        b /= 2;
    }
    return res;
}
void panda() {
    long double r;
    cin>>r;

        cout<<setprecision(2)<<fixed<<2*(3.14)*r<<"\n";



}
int main() {
    panda();
    return  0;
}
