解题：

#include<bits/stdc++.h>
#define ll long long
using namespace std;
long long a[100005];
int main(){
long long 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;
}