#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
//#define int ll
#define rep(i,a,b) for(int i = a; i < (b); i++)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define PB push_back
#define FS first
#define SD second
typedef pair<int, int> pii;
typedef vector<int> vi;
template<class X, class Y> X cmx(X &a, Y b) { a = max<X>(a, b); }
#define ary(k) array<int, k>

struct LL {
    LL *pref;
    LL *nex;
    double val;
    int type, id;
    LL() {}
    LL(double _val, int _type, int _id) : val(_val), type(_type), id(_id) {}
};

struct Event {
    double x, y1, y2;
    int type, id;
    bool operator< (Event e) const {
        return x < e.x;
    }
};

double licz(LL *start, LL *kon) {
    LL *aktu = start->nex;
    int cur = 0;
    double pop = 0, result = 0;
    while (aktu != kon) {
        if (aktu->type == 1) {
            if (cur == 0)
                pop = aktu->val;
            cur++;
        } else {
            cur--;
            if (cur == 0) {
                result += aktu->val - pop;
            }
        }
        aktu = aktu->nex;
    }
    return result;
}

void rem(LL *start, LL *kon, int id) {
    LL *aktu = start->nex;
    while (aktu != kon) {
        if (aktu->id == id) {
            aktu->pref->nex = aktu->nex;
            aktu->nex->pref = aktu->pref;
        }
        aktu = aktu->nex;
    }
}

void add(LL *start, LL *kon, LL *added) {
    LL *aktu = start;
    while (aktu != kon) {
        if (aktu->val < added->val && !(aktu->nex->val < added->val)) {
            added->nex = aktu->nex;
            added->pref = aktu;
            aktu->nex->pref = added;
            aktu->nex = added;
            break;
        }
        aktu = aktu->nex;
    }
}

signed main() {
	cin.tie(0)->sync_with_stdio(0); cin.exceptions(cin.failbit);
    while (true) {
        int n;
        cin >> n;
        if (n == 0)
            break;
        vector<Event> events;
        for (int i = 0; i < n; i++) {
            double x1, y1, x2, y2;
            cin >> x1 >> y1 >> x2 >> y2;
            events.PB({x1, y1, y2, 1, i});
            events.PB({x2, y1, y2, -1, i});
        }
        sort(all(events));
        LL *start = new LL();
        LL *kon = new LL();
        start->nex = kon;
        kon->pref = start;
        start->val = -2000;
        kon->val = 2000;
        double prev = -2000, res = 0;
        for (auto event : events) {
            res += licz(start, kon) * (event.x - prev);
            prev = event.x;
            if (event.type == 1) {
                LL *add1 = new LL(event.y1, 1, event.id);
                LL *add2 = new LL(event.y2, -1, event.id);
                add(start, kon, add1);
                add(start, kon, add2);
            } else {
                rem(start, kon, event.id);
            }
        }
        cout << res << ".00\n";
    }
}