20->0

#include<iostream>
using namespace std;
double n,a[2003],b[2003],c[2003],sum1,sum2;
double p1,p2;
double ans,ansx;
void dfs(int cnt){
    if(cnt==n+1){
        for(int i=1;i<=n;i++) sum1+=c[i];
        p1=1.00*sum1/n;
        for(int i=1;i<=n;i++) ans+=(c[i]-p1)*(c[i]-p1);
        ansx=max(ans/n*n*n,ansx);
        return;
    }
    c[cnt]=a[cnt],dfs(cnt+1);
    c[cnt]=b[cnt],dfs(cnt+1);
    return;
}
int main(){
    cin>>n;
    for(int i=1;i<=n;i++) cin>>a[i];
    for(int i=1;i<=n;i++) cin>>b[i];
    dfs(1);
    cout<<ansx;
}

10->0

#include<iostream>
using namespace std;
double n,a[20003],b[20003],sum1,sum2;
double p1,p2;
double ans;
int main(){
    cin>>n;
    for(int i=1;i<=n;i++) cin>>a[i],sum1+=a[i];
    p1=sum1/n;
    for(int i=1;i<=n;i++) cin>>b[i],sum2+=b[i];
    p2=sum2/n;
    for(int i=1;i<=n;i++) ans+=(a[i]-p1)*(a[i]-p1);
    cout<<ans/n*n*n;
}