#pragma GCC optimize(2)
#pragma G++ optimize(2)
#include<bits/stdc++.h>
#include<math.h>
#define si(a) scanf("%d",&a)
#define sl(a) scanf("%lld",&a)
#define sd(a) scanf("%lf",&a)
#define ss(a) scanf("%s",a)
#define ms(a) memset(a,0,sizeof(a))
#define repi(i,a,b) for(register int i=a;i<=b;++i)
#define repd(i,a,b) for(register int i=a;i>=b;--i)
#define reps(s) for(int i=head[s];i;i=e[i].nxt)
#define ce(i,r) i==r?'\n':' '
#define pb push_back 
#define fi first
#define se second
#define pr(x) cout<<#x<<": "<<x<<endl

using namespace std;
typedef long long ll;

double pi = acos(-1);
int r;

int main(){
	cin>>r;
	cout<<2 * r * pi;
	return 0;
} 