/*
ID: wangyuxi20040901
TASK:  ()
LANG: C++
DATE: 20200419 19:22:44
*///using CRLF, UTF-8

#include <cstdio>
#include <algorithm>
#include <cstring>
#define pr printf
#define F(i, j, k) for(register int i = j, kkllkl = k; i <= kkllkl; ++i)
#define G(i, j, k) for(register int i = j, kkllkl = k; i >= kkllkl; --i)
#define clr(a, v) memset((a), v, sizeof(a))
using namespace std;
typedef long long ll;

#define isd(x) (('0' <= (x) && (x) <= '9') || (x) == '-')
int rd() {
    int ans = 0, sign = 1; char c = getchar();
    while(!isd(c)) c = getchar();
    if(c == '-') sign = -1, c = getchar();
    while(isd(c)) ans = (ans << 3) + (ans << 1) + c - '0', c = getchar();
    return sign == 1 ? ans : -ans;
}

/* --------------------------CSYZ1921------------------------- */
#include <cmath>
int r;
double ans = acos(-1);

int main() {
    // freopen(".in", "r", stdin);
    // freopen(".out", "w", stdout);
    r = rd();
    ans *= r;
    pr("%.20lf\n", ans);
    return 0;
}
/*
*/