#include <iostream>
using namespace std;

int fibonacci_sum(int n) {
    if (n <= 0) return 0;
    if (n == 1) return 1;
    
    int a = 1, b = 1;  // 初始化为斐波那契数列的前两项
    int sum = 2;       // 前两项的和
    
    for (int i = 3; i <= n; i++) {
        int c = a + b;
        sum += c;
        a = b;
        b = c;
    }
    
    return sum;
}

int main() {
    int n;
    cin >> n;
    cout << fibonacci_sum(n);
    return 0;
}
/**************************************************************
	Problem: 1238
	User: panyuchen
	Language: C++
	Result: Wrong Answer
****************************************************************/