#include "bits/stdc++.h"
using namespace std;

#define REP(i, n) for(int i=0; i<(n); i++)
#define FOR(i, a, b) for(int i=(a);i<(b);i++)

int N;
vector<long double> A(200);
signed main()
{
    cin >> N;
    A[0] = 4.0;
    A[1] = 3.0;
    FOR(i,2,N+1) {
        A[i] = (19 * A[i-1] - 12 * A[i-2]) / 4.0;
    }
    printf("%.9Lf\n", A[N]);
    return 0;
}