#include<bits/stdc++.h>

using namespace std;

void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}

template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define ANTHRO(x...) cerr << "[" << #x << "] = [", _print(x)
#else
#define ANTHRO(x...)
#endif


#define ll long long
#define MOD 1000000007
#define INF (ll)1e18
#define EPS 1e-9
/*
	XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
    No, nothing good starts in a getaway car
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
*/

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    long double pi=3.1416;
    long double r; cin>>r;
    cout<<setprecision(5)<<(long double)(2*pi*r)<<endl;
    return 0;
}
