题解

#include <iostream>
using namespace std;
long long a[25];
int main(){
    int n;
    cin >> n;
    a[1] = 1;
    a[2] = 2;
    for(int i = 3;i <= n;i++){
        a[i] = a[i - 1] + a[i - 2];
    }
    cout << a[n];
    return 0;
}