#include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>
using namespace std;

#define RREP(i,s,e) for (i = s; i >= e; i--)
#define rrep(i,n) RREP(i,(int)(n)-1,0)
#define REP(i,s,e) for (i = s; i <= e; i++)
#define rep(i,n) REP(i,0,(int)(n)-1)
#define INF 100000000

typedef long long ll;

int main() {
    int i, n;
    double a[101];
    cin >> n;
    a[0] = 4; a[1] = 3;
    REP (i,2,n)
        a[i] = a[i-1] * 19 / 4 - a[i-2] * 3;
    cout << fixed << setprecision(10) << a[n] << endl;
    return 0;
}