URAL 1225 - Flags

//aarifshuvo   ``CSEJU

#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define SZ(x) ((int)(x).size())
#define scl(x) scanf("%lld", &x)
#define scll(x,y) scanf("%lld %lld", &x, &y)
#define all(x) (x).begin(),(x).end()
#define mem(a,d) memset(a,d,sizeof a)
#define REP(i,n) for(int i=0;i<n;i++)
#define REV(i,n) for(int i=n-1;i>=0;i--)
#define FOR(i,a,b) for(int i=a;i<=b;i++)
#define pri(a) cout<<a<<endl
#define prii(a,b) cout<<a<<" "<<b<<endl
using namespace std;

ll dp[50][4][4];
ll n;

ll go(ll i, ll cur, ll prev)
{
    if(i==n-1)
    {
        if(cur==2) return 0;
        else        return 1;
    }
    ll &ret = dp[i][cur][prev];
    if(~ret)        return ret;

    ret = 0;

    if(cur==1)
    {
        ret += ( go(i+1,3,cur) + go(i+1,2,cur) );
    }
    else if(cur==3)
    {
        ret += ( go(i+1,1,cur) + go(i+1,2,cur) );
    }
    else if(cur==2)
    {
        if(prev==1) ret += go(i+1,3,cur);
        else        ret += go(i+1,1,cur);
    }
    return ret;
}

// 1 white 2 blue 3 red

int main()
{
    scl(n);
    mem(dp,-1);
    ll ans = go(0,1,0) + go(0,3,0);
    printf("%lld\n", ans);
    return 0;
}