            #include<bits/stdc++.h>
            using namespace std;
            #define rep(i,b) for(i=0;i<b;++i)
            #define repp(i,a,b) for(i=a;i<b;++i)
            #define endl "\n"
            //#define LLONG_MAX 99999999999999
            typedef long long int lli;
            int main(){
                ios_base::sync_with_stdio(false);
                cin.tie(NULL);
                lli t,n,i,j,b,a,q,m;
                long double r;
                cin>>r;
                cout<<2.0*3.141*r<<endl;

            }

