#include <iostream>
#include <cstdio>
#include <bits/stdc++.h>
#include <math.h>
#include <string>
using namespace std;

// === Debug macro starts here ===============================

void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V>
void __print(const pair<T, V> &x)
{
    cerr << '{';
    __print(x.first);
    cerr << ',';
    __print(x.second);
    cerr << '}';
}
template <typename T>
void __print(const T &x)
{
    int f = 0;
    cerr << '{';
    for (auto &i : x)
        cerr << (f++ ? "," : ""), __print(i);
    cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v)
{
    __print(t);
    if (sizeof...(v))
        cerr << ", ";
    _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)               \
    cerr << "[" << #x << "] = ["; \
    _print(x)
#else
#define debug(x...)
#endif

// === Debug macro ends here =================================

#define F first
#define S second
#define ll long long
const int MOD = 1e9 + 7;
const float epsilon = 0.0005f;
const double eps = 1e-9;
const float PI = 3.1415926535897932384626433f;

vector<pair<int, int>> dire{{0, 1}, {0, -1}, {1, 0}, {-1, 0}};

// std::cout << std::fixed;
// std::cout << std::setprecision(9);

void solve()
{
    int a,b,c,d;
    cin>>a>>b>>c>>d;
    cout << a * d - b * c;
}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    // int t;
    // cin >> t;
    // for (int i = 0; i < t; ++i)
    // {
    //     solve();
    // }
    solve();
    return (0);
}

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define LLMAX (1ll << 60) - 1
#define INTMAX (1 << 30) - 1
#define MOD 1000000007 
#define NMAX 1000*100+1

#define numberOfSetBits(S) __builtin_popcount(S) // __builtin_popcountl(S) __builtin_popcountll(S)
#define MSET(x,y) memset(x,y,sizeof(x))
#define gcd(a,b) __gcd(a,b)
#define all(x)  x.begin(),x.end()
#define isOn(S, j) (S & (1 << j))
#define endl '\n'
#define setBit(S, j) (S |= (1 << j))
#define clearBit(S, j) (S &= ~(1 << j))
#define toggleBit(S, j) (S ^= (1 << j))
#define print(x)    for(auto it:x) cout<<it<<' '; cout<<endl;
#define printii(x)  for(auto it:x) cout<<it.F<<' '<<it.S<<'\t';  cout<<endl;
#define in(x,n)   for(int e=0;e<n;e++){ll y;cin>>y;x.pb(y);}

#define vi vector<ll>
#define vvi vector<vi>
#define ii pair<ll,ll>
#define pll pair<ll,ll>
#define vii vector<ii>
#define vvii vector<vii>
#define viii vector<pair<ii,ll>>
#define pb push_back
#define F first
#define S second
#define mp make_pair
#define mc(a,b,c) mp(mp(a,b),c)


int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    ll a,b,c,d;
    cin>>a>>b>>c>>d;
    cout<<a*d-b*c;    

    return 0;
}