这是怎么回事

#include<bits/stdc++.h>
using namespace std;
long long n,p,a[10000001] = {0,1};
int main(){
	//freopen(".in","r",stdin);
	//freopen(".out","w",stdout);
	
	cin>>n;
	cin>>p;
	for(int i = 2;i<=n;i++){
		a[i] = (a[i-1]+a[i-2])%p;
	}
	cout<<a[n];
	return 0;
}