#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <utility>
#include <vector>
using namespace std;

#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define sz(x) (int((x).size()))
#define bit(x) (1 << (x))
#define cnt1(x) (__builtin_popcount(x))

template<class T>
inline void chkmax(T& x, T y) {
    if (x < y) x = y;
}

template<class T>
inline void chkmin(T& x, T y) {
    if (y < x) x = y;
}

typedef long long LL;
typedef double DB;
typedef pair<int, int> PII;
typedef vector<int> VI;
const int MX = 65;
LL dp[MX];

void init() {
    int i;
    
    dp[0] = 1, dp[1] = 2, dp[2] = 3;
    for (i = 3; i < MX; i++) dp[i] = dp[i - 1] + dp[i - 3];
}

int main() {
    
    int n;
    
    init();
    while (scanf("%d", &n) == 1) printf("%I64d\n", dp[n] - 1);
    return 0;
}