#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<vector>
#include<queue>
#include<string>
#include<map>
#include<stack>
#include<iostream>
#include<stdlib.h>
using namespace std;

typedef long long ll;

ll a[70];

int main()
{
    int i,n;

    a[0]=1,a[1]=2,a[2]=3;
    for(i=3;i<=61;i++)
    {
        a[i]=a[i-1]+a[i-3];
    }
    while(scanf("%d",&n)!=EOF)
    {
        printf("%I64d\n",a[n]-1);
    }
    return 0;
}