#include<bits/stdc++.h>
using namespace std;
 
#define SPEED ios::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define read()          freopen("in.txt","r",stdin)
#define write()         freopen("out.txt","w",stdout)
#define sf(n)           scanf("%d",&n)
#define sfd(n)          scanf("%lf",&n)
#define sl(x)           scanf("%I64d",&x)
#define sfl(n)          scanf("%lld", &n)
#define sfc(n)          scanf(" %c",&n)
#define sful(n)         scanf("%llu",&n)
#define ull             unsigned long long
#define endll           pf("\n")
#define pf              printf
#define PI              acos(-1.0)
#define eps             1e-12
#define mem(a,b)        memset(a,b,sizeof(a))
#define pb              push_back
#define xx              first
#define yy              second
#define pii             pair<int, int>
#define pll             pair<ll, ll>
#define MP make_pair
#define ll              long long
#define uniq(a)      a.erase(unique(a.begin(), a.end()), a.end())
#define cnd tree[idx]
#define lnd tree[idx<<1]
#define rnd tree[(idx<<1)+1]
#define lndp (idx<<1),(b),((b+e)>>1)
#define rndp ((idx<<1)+1),(((b+e)>>1)+1),(e)
#define inf 1999999999
#define MX 100005
#define MOD  1000000000
#define mod1 1000000009
#define var    int i,j,c=0,t,l,r,mid,z=0,k,x,y
 
//int dx[8]={0,0,1,1,-1,-1,1,-1};  //8 direction
//int dy[8]={1,-1,1,-1,1,-1,0,0};
//int dx[4]={1,-1,0,0};  //4 direction
//int dy[4]={0,0,1,-1};
ll n,m,a[200005];
 
int main()
{
    ll var;
    double d;
  
    sfd(d);
    d=2*PI*d;
    pf("%.7f\n",d);
}