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

typedef long long ll;
typedef long long ld;
typedef vector<int> vi;
typedef pair<ll, ll> pii;

struct Node
{
    int node, len;
    Node() {node = len = 0;}
    Node(int node, int len) {this -> node = node, this -> len = len;}
};
typedef vector<Node> vg;

#define MAX 1
#define NON 0
#define MOD 1000000007

#define FOR(type, i, a, b) for(type i = a; i < b; i++)
#define FORR(type, i, b, a) for(type i = b; i > a; i--)
#define FORE(type, i, a, b) for(type i = a; i <= b; i++)
#define FORRE(type, i, b, a) for(type i = b; i >= a; i--)

#define testBit(n, bit) ((n >> bit) & 1)
#define flipBit(n, bit) (n ^ (1ll << bit))

main()
{
    ios_base::sync_with_stdio(0); cin.tie(0);
    int r; cin >> r;
    cout << fixed << setprecision(5) << r * 2 * acos(-1);
}