B. Chris and Magic Square

#include<iostream>
#include<algorithm>
#include<cstdio>
#define ll  long long
using namespace std;
ll s1[550];
ll s2[550];
ll s3,s4;
ll v1,v2,v3;
int main(){
    ll n,a[505][505];
    cin>>n;
    int v;
    if(n==1){
        ll t;
        cin>>t;
        cout<<"1"<<endl;
        return 0;
    }
    ll x,y;
    ll top=0;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            cin>>a[i][j];
            if(!a[i][j]){
                x=i;
                y=j;    
            }
        }
    }
    ll sum=0;
    ll sumpre=0;
    for(int i=1;i<=n;i++)
    {
        sumpre+=a[x][i];
        if(x>1)
        sum+=a[1][i];   
        else if(x==1)
        sum+=a[n][i];
    }
    a[x][y]=sum-sumpre;

    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            s1[i]+=a[i][j];
            if(i==1){
                v1=s1[i];
            }
        }
        if(i>1){
            if(s1[i]!=s1[i-1]){
                cout<<-1<<endl;
                return 0;
            }
        }
    }
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            s2[i]+=a[j][i];
            if(i==1){
                v2=s2[i];
            }
        }
        if(i>1){
            if(s2[i]!=s2[i-1]){
                cout<<-1<<endl;
                return 0;
            }
        }
    }
    for(int i=1;i<=n;i++){
            s3+=a[i][i];
    }
    for(int i=1;i<=n;i++){
        s4+=a[i][n-i+1]; 
    }
    if(s3!=s4){
        cout<<-1<<endl;
        return 0;
    }else{
        v3=s3;
    }

    if(a[x][y]<=0){
        cout<<-1<<endl;
        return 0;
    }

    if(v1==v2&&v2==v3)
    cout<<a[x][y]<<endl;    
    else
    cout<<-1<<endl;
    return 0;
}