#include <stdio.h>
#include <math.h>
#define N 20
double  shulie(int n) 
{
    int  s[N] = { 0 }, i;
    double r1;
    s[0] = 0;
    s[1] = 0;
    s[2] = 1;
    for (i = 3; i < n; i++) {
        s[i] = s[i - 1] + s[i - 2] + s[i - 3];
    }
    r1 = 0;
    for (i = 0; i < n; i++) {
        r1 += sqrt((double)s[i]);
    }
    return r1;
}
int main()
{
    int n;
    double r;
    scanf("%d", &n);
    r = shulie(n);
    printf("%f", r);
    return 0;
}