Hoax or what(multiset的应用)

#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
const int N=1e5+5;
 
ll ans;
multiset<int>s;
multiset<int>::iterator np,mp;
int main()
{
   int n,k,v;
   while (~scanf("%d",&n)&&n){
       ans=0;s.clear();
       for (int i=0;i<n;i++){
            scanf("%d",&k);
            for (int j=0;j<k;j++) scanf("%d",&v),s.insert(v);
            np=s.begin(),mp=(s.end());mp--;
            ans+=*mp-*np;
            s.erase(mp); s.erase(np);
            
       }
       cout<<ans<<endl;
 
   }
}