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

#define ll long long int
#define endl '\n'
ll const mod = 1e9 + 7;

ll power(ll x, ll y, ll p){ //x^y % p
    ll res = 1;      // Initialize result 
    x = x % p;  // Update x if it is more than or equal to p 
    while (y > 0) {
        if (y & 1) // If y is odd, multiply x with result 
        res = (res*x) % p; 
        // y must be even now 
        y = y>>1; // y = y/2 
        x = (x*x) % p; 
    } 
    return res; 
}

ll MIN(ll a, ll b) {
    if(a<b) return a;   return b;
}

ll MAX(ll a, ll b) {
    if(a>b) return a;   return b;
}

int main() {
    
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    
    double r;
    cin>>r;
    
    cout<<fixed;
    cout<<setprecision(10)<<2*3.1415926535*r;
    
	// your code goes here
	return 0;
}
