	/*It is better to taste failure 
	  than to regret*/
		
            #include <algorithm>
        #include <bitset>
        #include <cassert>
        #include <chrono>
        #include <cmath>
        #include <complex>
        #include <cstdio>
        #include <cstdlib>
        #include <cstring>
        #include <ctime>
        #include <deque>
        #include <functional>
        #include <iomanip>
        #include <iostream>
        #include <iterator>
        #include <limits>
        #include <climits>
        #include <list>
        #include <map>
        #include <numeric>
        #include <queue>
        #include <random>
        #include <ratio>
        #include <set>
        #include <sstream>
        #include <stack>
        #include <string>
        #include <unordered_map>
        #include <unordered_set>
        #include <utility>
        #include <vector>
        using namespace std;
         
        typedef long long ll;
        typedef long double LD;
        #define int ll
        #define double LD
        #define pb push_back
        #define mp make_pair
        #define REP(i,n) for (int i = 0; i < n; i++)
        #define loop(i,n) for (int i = 0; i < n; i++)
        #define FOR(i,a,b) for (int i = a; i < b; i++)
        #define REPD(i,n) for (int i = n-1; i >= 0; i--)
        #define FORD(i,a,b) for (int i = a; i >= b; i--)
        #define remax(a,b) a = max(a,b)
        #define remin(a,b) a = min(a,b)
        #define mem(a, b) memset(a, b, sizeof a)
        #define append push_back
        #define all(v) v.begin(),v.end()
        #define F first
        #define S second
        #define print(x) cout << (x) << endl
        #define PRECISION(x) cout << fixed << setprecision(x)
        #define FAST_IO ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)
        #define Test(){int t;cin >> t;for(int i=1;i<=t;i++){cout << "Case #" << i << ": ";solve();}}
        #define test(){int t; cin>>t; for(int i=1;i<=t;i++){solve();}}
        #define vi vector<int>
        #define vii vector<pair<int,int> > 
        #define vci vector<pair<char,int> >
        #define vic vector<pair<int,char> > 
        #define N 200005
        #define pb push_back
        const double PI=acos(-1);
        const int MOD = 1000000007;
        const int FMOD = 998244353;
        
        void solve()
        {
        	int a,b,c,d;
        	cin>>a>>b>>c>>d;
        	
        	cout<<a*d-b*c<<endl;
		}
        signed main()
        {
        //	test();
        solve();
		}
#include<iostream>

using namespace std;
int main()
{
    int a,b,c,d;
    cin>>a>>b>>c>>d;
    cout<<a*d-b*c;
    return 0;
}