#include <iostream>
using namespace std;
int main(void){
    // Your code here!
    int a,b,c,d;
    
    cin >> a >> b >> c >> d;
    
    int ans = a * d - b * c;
    
    cout << ans;

}


//#include <atcoder/maxflow.hpp>
#include <memory>
#include <iostream>
#include <map>
#include <list>
#include <set>
#include <algorithm>
#include <vector>
#include <sstream>
#include <string>
#include <functional>
#include <queue>
#include <deque>
#include <stack>
#include <limits>
#include <unordered_map>
#include <unordered_set>
#include <cmath>
#include <fstream>
#include <iterator>
#include <random>
#include <chrono>
#include <complex>
#include <thread>

#define forr(i,start,count) for (int i = (start); i < (start)+(count); ++i)
#define set_map_includes(set, elt) (set.find((elt)) != set.end())
#define readint(i) int i; cin >> i
#define readll(i) ll i; cin >> i
#define readdouble(i) double i; cin >> i
#define readstring(s) string s; cin >> s

typedef long long ll;

using namespace std;
//using namespace atcoder;

//ll modd = (1000LL * 1000LL * 1000LL + 7LL);
ll modd = 998244353;


int main()   {

    ios_base::sync_with_stdio(false);

    cout.precision(17);
    mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
    uniform_int_distribution<int> rand_gen(0, modd);   // rand_gen(rng) gets the rand no

    auto start = chrono::steady_clock::now(); 

//    readint(test_cases);
    int test_cases = 1;
    forr(t, 1, test_cases) {
        readint(a); readint(b); readint(c); readint(d);
        cout << a*d-b*c << endl;
    }
//        auto stop = chrono::steady_clock::now(); 
//        auto duration = chrono::duration_cast<chrono::milliseconds>(stop - start);
//        cout << "Duration: " << duration.count() << endl;
    return 0;
}

