C :

#include<stdio.h>
#define max(a,b)	(a>b?a:b)
int n;
int a,b,c,x[100010];
long long f[100010];
int main()
{
	scanf("%d %d %d %d",&n,&a,&b,&c);
	for(int i=1;i<=n;i++)
	{
		int tmp=((long long)a*i*i+b*i+c)%20000;
    	x[i]=tmp-10000;
	}
	f[n]=x[n];
	for(int i=n-1;i>=0;i--)
	f[i]=x[i]+max(f[i+1],f[i+2]);
	printf("%lld",f[0]);
	return 0;
} 

C++ :

#include<bits/stdc++.h>
using namespace std;
int x[100010],n,A,B,C;
int main(){
	cin>>n>>A>>B>>C;
	for (int i = 1; i <= n; i++){
    	int tmp = ((long long)A * i * i + B * i + C) % 20000;
    	x[i] = tmp - 10000;
    }
    for (int i = 2; i <= n+1; i++){
    	x[i] = max(x[i-1],x[i-2]) + x[i];
    }
    cout<<x[n+1];
	return 0;
}