#include<cstdio>
#include<iostream>
#include<cstring>
#include<iomanip>
#include<set>
#include<ctime>
#include<vector>
#include<queue>
#include<map>
#include<algorithm>
using namespace std;
__int64 a[61];
int main()
{
//    freopen("D://input.txt","r",stdin);
//    freopen("D://output.txt", "w", stdout);
    int n;
    a[1]=1;
    a[2]=2;
    a[3]=3;
    for (int i=4;i<=60;i++)
        a[i] = a[i - 1] + a[i-3]+1;
    while (scanf("%d", &n) != EOF)
    {
        printf("%I64d\n",a[n]);
    }
//    printf("%.6lf\n",(double)clock()/CLOCKS_PER_SEC);
    return 0;
}