#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for(int i=0;i<(n);i++)
#define N 4050
#define INF 1000000000
typedef long long ll;
#define ALL(v) (v).begin(),(v).end()
#define SZ(x) int(x.size())
#define IN(a) cin>>(a)
#define OUT(a) cout<<(a)<<endl
typedef pair<int,int> P;
const int MAX = 1000000;
const int MOD = 1000000007;
int main(){
    int r;
    cin>>r;
    double ans=(double)2*r*3.141592;
    printf("%.8lf\n",ans);
    return 0;
}