#include<iostream>
#include<cstdio>
#include<cstring>
#define REP(i,e,s) for(register int i=e; i<=s; i++)
#define DREP(i,e,s) for(register int i=e; i>=s; i--)
#define DE(...) fprintf(stderr,__VA_ARGS__)
#define DEBUG(a) DE("DEBUG: %d\n",a)
#define file(a)	freopen(a".in","r",stdin);freopen(a".out","w",stdout)
using namespace std;

int read() {
	int x=0,f=1,ch=getchar();
	while(ch>'9'||ch<'0'){if(ch=='-')f=-1;ch=getchar();}
	while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
	return x*f;
}

int main() {
	int r=read();
	printf("%.8lf\n",2*r*3.1415926);
	return 0;
}

