Hoax or what

#include<bits/stdc++.h>

using namespace std;

int main()
{
    long long n,k,i,ans,a;

    while(scanf("%lld",&n)&&n)
    {
        ans = 0;

        multiset<long long>multist;

        while(n--)
        {
            scanf("%lld",&k);

            for(i=1; i<=k; i++)
            {
                scanf("%lld",&a);

                multist.insert(a);
            }

            multiset<long long>::iterator mn,mx;

            mn = multist.begin();

            mx = multist.end();

            mx--;

            ans += ( *mx - *mn );

            multist.erase(mn);

            multist.erase(mx);
        }

        printf("%lld\n",ans);
    }

    return 0;
}