C++ 代码

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

int F(int n){
    if(n == 1)return 2;
    else if(n == 2) return 3;
    else return F(n-1) + F(n-2);
}

int main()
{
    int n;
    cin >> n;

    cout << F(n) << endl;

    return 0;
}