思路：

代码：

#include<bits/stdc++.h>
using namespace std;
int main(){
    int a, b1, b2, month;
    while(cin>>month){
        a = 0;
        b1 = 1;
        b2 = 0;
        
        int olda, oldb1, oldb2;
        for (int i = 0; i < month-1; i++)
        {
            olda = a;
            oldb1 = b1;
            oldb2 = b2;
            a = olda + oldb2;
            b1 = olda + oldb2;
            b2 = oldb1;
        }
        cout << a + b1 + b2 << endl;
    }

    return 0;
}