#include<bits/stdc++.h>

#define ll long long int
#define ld long double
#define pb push_back
#define eb emplace_back
#define xx first
#define yy second
#define INF 0x3f3f3f3f // 100'000'000'000'000
#define all(x) x.begin(),x.end()
#define show(x) cerr<<#x<<" : "<<x<<'\n';
#define yogeshk972() cerr<<"\nTime Taken : "<<(float)(clock()-time_p)/CLOCKS_PER_SEC<<"\n";
clock_t time_p=clock();

using namespace std;
const int N= 1e5+10, mod=1e9+7;

int main(){
    ios_base::sync_with_stdio(false);cin.tie(NULL);  

    double r;
    cin>>r;
    cout<<fixed<<setprecision(10)<<(2*acos(-1)*r);
    
    return 0;
}

