set UVA 11136 Hoax or what

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;

int main(void)  {
    int n;
    multiset<int> S;
    while (scanf ("%d", &n) == 1)   {
        if (!n) break;
        S.clear ();
        ll ans = 0;
        for (int m, i=1; i<=n; ++i)    {
            scanf ("%d", &m);
            for (int v, j=1; j<=m; ++j)    {
                scanf ("%d", &v);
                S.insert (v);
            }
            int p1 = *(S.begin ());
            int p2 = *(S.rbegin ());
            ans += p2 - p1;
            S.erase (S.find (p1));
            S.erase (S.find (p2));
        }
        printf ("%I64d\n", ans);
    }

    return 0;
}