UVA 11136 Hoax or what——multiset

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <set>
using namespace std;
int main() {
    int n, k, x;
    while (~scanf("%d", &n) && n) {
        long long ans = 0;
        multiset<int> s;
        for (int i = 0; i < n; i++) {
            scanf("%d", &k);
            for (int j = 0; j < k; j++) {
                scanf("%d", &x);
                s.insert(x);
            }
            multiset<int>::iterator it1, it2;
            it1 = s.begin(); it2 = --s.end();
            ans = ans + (*it2) - (*it1);
            s.erase(it1);
            s.erase(it2);
        }
        cout << ans << endl;
    }
    return 0;
}