#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb push_back
#define en cout<<'\n'
#define vec vector<ll>
#define vecs vector<string>
#define pii pair<ll,ll>
#define VP vector< pii >
#define all(v) (v).begin(),(v).end()
#define ff first
#define ss second
#define mm " "
#define MM " "
#define f(i,a,b) for(ll i=(ll)a;i<(ll)b;i++)
#define rf(i,a,b) for(ll i=(ll)a;i>=(ll)b;i--)
#define fa(x,v) for(auto x:v)
#define INF (ll)(9e18)
#define Sort(v) sort(v.begin(),v.end())
#define test ll T;cin>>T;while(T--)
#define fast ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define db1(x) cout<<#x<<"="<<x<<'\n'
#define db2(x,y) cout<<#x<<"="<<x<<","<<#y<<"="<<y<<'\n'
#define db3(x,y,z) cout<<#x<<"="<<x<<","<<#y<<"="<<y<<","<<#z<<"="<<z<<'\n'
#define max3(a,b,c) max(a,max(b,c))
#define min3(a,b,c) min(a,min(b,c))
#define max4(a,b,c,d) max(a,max3(b,c,d))
#define min4(a,b,c,d) min(a,min3(b,c,d))
#define lb(v,val)  (lower_bound(v.begin(),v.end(),val)-v.begin())
#define ub(v,val)  (upper_bound(v.begin(),v.end(),val)-v.begin())
 const ll MOD=1000000000+7;
const ll MOD1 = 998244353;
string itos(ll s)
{
string str=to_string(s);
return str;
}
ll powerm(ll a, ll b, ll MOD) {
    a %= MOD;
    ll res = 1;
    while (b > 0) {
        if (b & 1)
            res = res * a % MOD;
        a = a * a % MOD;
        b >>= 1;
    }
    return res;
}

ll power(ll a, ll b) {

  ll res = 1;
    while (b > 0) {
        if (b & 1)
            res = res * a ;
        a = a * a ;
        b >>= 1;
    }
    return res;
}

bool ispalindrome(ll len,string str)
{

    for(int i = 0; i < len/2; i++){
        if(str[i] != str[len-1-i]){
            return false;
        }
    }
    return true;
}
ll binarySearch(ll a[],ll low,ll high,ll key){
    while(low<=high)
    {
        ll mid=(low+high)/2;
        if(a[mid]<key)
        {
            low=mid+1;
        }
        else if(a[mid]>key)
        {
            high=mid-1;
        }
        else
        {
            return mid;
        }
    }
    return -1;
}
ll isprime(ll N){
    if(N<2 || (!(N&1) && N!=2))
        return 0;
    for(ll i=3; i*i<=N; i+=2){
        if(!(N%i))
            return 0;
    }
    return 1;
}
vec v;
void factors(ll n)
{

for(ll i=1;i<=sqrt(n);i++)
{
    if(n%i==0)
    {
        if(i==n/i)
        v.pb(i);
        else
        {
            v.pb(i);
            v.pb(n/i);
        }

    }
}

sort(all(v));
}
ll gcd(ll a, ll b) {
   if (b == 0)
      return a;
   return gcd(b, a % b);
}
ll lcm(ll a,ll b)
{
	return (a/gcd(a,b))*b;
}

ll binarytodecimal(string n)
{
    string num = n;
    ll dec_value = 0;

   ll base = 1;

   ll len = num.length();
    for (int i = len - 1; i >= 0; i--) {
        if (num[i] == '1')
            dec_value += base;
        base = base * 2;
    }

    return dec_value;
}


vec seive;
void Seive() {
    const ll maxn = 1e5 + 5;
    seive.resize(maxn);
    f(i,0, maxn) seive[i] = i;
    for(ll i=2;i<=maxn;i+=2)
    seive[i] = 2;
    seive[1] = -1;
    seive[0] = -1;
    for (ll i = 3; i <= maxn; i += 2)
        if (i == seive[i])
        for (ll j = i * i ; j < maxn ; j += i)
        if (seive[j] == j)
        seive[j] = i;
}



int main()
{
fast;

ll H,W,h,w;
cin>>H>>W>>h>>w;

cout<<H*W-h*W-w*H+h*w;
















}


Measured memory_bytes: 256