Who can help me?

#include<bits/stdc++.h>
using namespace std;
unsigned long long a[5678910];
unsigned long long work(unsigned long long n){
    if(a[n]!=0)return a[n];
    if(n==1)return 1;
    if(n==2)return 2;
    a[n]=work(n-1)+work(n-2);
    return a[n];
}
int main() {
    unsigned long long n;
	cin>>n;
    cout<<work(n);
     return 0;
}