#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;

long long f[70];

int main()
{
    memset(f, 0, sizeof(f));
    f[0] = 0; f[1] = 1; f[2] = 2; f[3] = 3;
    for (int i = 4; i <= 60; i++)
        f[i] = f[i-1] + f[i-3] + 1;
    int n;
    while (scanf("%d", &n) != EOF)
        printf("%I64d\n", f[n]);
    return 0;
}