#include <bits/stdc++.h>
using namespace std;
#define mod 1000000007
#define int long long int
#define pb push_back
#define mk make_pair
#define flp(i, k, n) for(int i=k; i<n; i++)
#define rev(i, k) for(int i=k; i>=0; i--)
#define F first
#define S second
#define IOS ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
int dx[4] = {-1,1,0,0};
int dy[4] = {0,0,-1,1};

int32_t main(void)
{
    int r;
    cin>>r;
    r*=2;
    cout<<(double)(r)*(double)(3.14)<<"\n";
    return 0;
}