#include <bits/stdc++.h>

using namespace std;

#define M (ll)(1e9+7)
#define EPS (double)(1e-12)
#define ll long long
#define N (ll)(2*1e5)

class Graph {
    
    public:
        int V;
        vector<int> *adj;
        bool *visited;

        Graph(int V) {
            this->V = V;
            adj = new vector<int>[V];
            visited = new bool(V);
            memset(visited, 0, V*sizeof(bool));
        }
        void addEdge(int u, int v) {
            adj[u].push_back(v);
            adj[v].push_back(u);
        }
        void dfs(int u);
        void bfs(int u);
};

void Graph::dfs(int u) {
    if(visited[u])
        return;
    visited[u] = true;
    cout << u << " ";

    for(auto a : adj[u]) {
        dfs(a);
    }
}

void Graph::bfs(int u) {
    queue<int> fila;

    fila.push(u);
    visited[u] = true;
    while(!fila.empty()) {
        int s = fila.front();
        cout << s << " ";
        fila.pop();

        for(auto a : adj[s]) {
            if(!visited[a]) {
                visited[a] = true;
                fila.push(a);
            }
        }
    }
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    double res;
    int value;

    cin >> value;

    res = (double)2*value*(double)M_PI;

    cout << setprecision(10) << fixed << res << endl;

    return 0;
}