#include <bits/stdc++.h>
using namespace std;

#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,n,a) for (int i=n-1;i>=a;i--)
#define all(cont) cont.begin(), cont.end()
#define rall(cont) cont.end(), cont.begin()
#define SZ(x) ((x).size())
#define i0(x) memset(x,0,sizeof(x))
#define i1(x) memset(x,-1,sizeof(x))
#define el cout<<endl
#define mp make_pair
#define pb push_back
#define X first
#define Y second
#define MOD 1000000007
#define N 100000
#define M 500000

#define trace1(x)                    cout <<#x<<": "<<x<< endl;
#define trace2(x, y)                 cout <<#x<<": "<<x<<" | "<<#y<<": "<<y<< endl;
#define trace3(x, y, z)              cout <<#x<<": "<<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl;
#define trace4(a, b, c, d)           cout <<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<endl;

typedef long long ll;
typedef long double lld;

template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

ll powmod(ll a,ll b) {ll res=1;a%=MOD; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%MOD;a=a*a%MOD;}return res;}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(NULL);
    ll n;
    cin>>n;
    lld nn=d;
    cout<<(nn*6.28318530717958623200);
    return 0;
}