#include <bits/stdc++.h>
using namespace std;
#define int         long long
#define endl        "\n"
#define P           pair<int, int>
#define dint        long double

const int mod = 998244353;

inline int add(int a, int b) { int res = a + b; if(res >= mod) res -= mod;  return res; }
inline int sub(int a, int b) { int res = a - b; if(res < 0) res += mod; return res;}
inline int mul(int a, int b) { return (a * b)%mod; }

int modpow(int a, int b) {
    if(a == 0 || a == 1 || b == 1) return a;
    int ans = 1;
    while(b) {
        if(b%2) ans = mul(ans, a);    
        b /= 2; a = mul(a, a);
    }   return ans;
}

void solve() {
    dint pi = 3.141592653589;
    int n; cin >> n;
    dint ans = 2*pi*n;
    cout << ans << endl;
}

signed main() {

    ios_base::sync_with_stdio(false);
    cin.tie(0); cout.tie(0);
    cout << fixed << setprecision(12);

    int T = 1;
    //cin >> T;

    while(T--) {
        solve();
    }
    return 0;

}

