样例3错了

方程对吗？哪里错了

#include<bits/stdc++.h>
#define int long long
using namespace std;
inline int read(){
    int x=0,f=1;char c=getchar();
    while(c<'0'||'9'<c){if(c=='-')f=-1;c=getchar();}
    while('0'<=c&&c<='9'){x=(x<<1)+(x<<3)+c-'0';c=getchar();}
    return x*f;
}
const int N=5e3+5;
int n,ans;
int a[N],f[N],sum[N];
signed main(){
    n=read();
    for(int i=1;i<=n;i++){
        a[i]=read();
        sum[i]=sum[i-1]+a[i];
        f[i]=sum[i-1];
    }
    for(int i=1;i<=n;i++)
        for(int j=1;j<i;j++)
            f[i]=max(f[i],f[j]+sum[(i-j-1)/2]*2+(((i-j-1)&1)?a[(i-j-1)/2+1]:0));
    for(int i=1;i<=n;i++)ans=max(ans,f[i]+sum[n-i]);
    printf("%lld",ans);
    return 0;
}