#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <stack>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;

int main(){
  vector<double> v;
  v.push_back(4);
  v.push_back(3);
  double p = 4;
  double q = 3;
  REP(i,2,101){
    double a = (19.0 * q - 12.0 * p)/4.0;
    v.push_back(a);
    p = q;
    q = a;
  }

  int N;
  cin >> N;
  cout << v[N] << endl;
  
  return 0;
}