Problem:
Score : 100 points

Problem Statement

There are H rows and W columns of white square cells.

You will choose h of the rows and w of the columns, and paint all of the cells contained in those rows or columns.

How many white cells will remain?

It can be proved that this count does not depend on what rows and columns are chosen.

Constraints

All values in input are integers.

1 \leq H, W \leq 20

1 \leq h \leq H

1 \leq w \leq W

Input

Input is given from Standard Input in the following format:

H W
h w

Output

Print the number of white cells that will remain.

Sample Input 1

3 2
2 1

Sample Output 1

1

There are 3 rows and 2 columns of cells. When two rows and one column are chosen and painted in black, there is always one white cell that remains.

Sample Input 2

5 5
2 3

Sample Output 2

6

Sample Input 3

2 4
2 4

Sample Output 3

0

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

/*
    Converted from Scratch by scratch2cpp (https://github.com/yos1up/scratch2cpp).
*/
#include <iostream>
#include <stdlib.h>
#include <string>
#include <vector>
#include <algorithm>
#include <math.h>
#define debug cerr << "--" << __LINE__ << "--" << "\n"
typedef long long ll;
using namespace std;

const double EPS = 1e-8;

class Var{ // NOTE: immutable
public:
    string sval;
    double dval;
    enum VarType {STRING = 0, NUMBER = 1};
    VarType type;
    enum NumericState {UNKNOWN = -1, STRINGY = 0, NUMERIC = 1};
    mutable NumericState numericState;

    Var(){sval = ""; type = STRING; numericState = STRINGY;} // represent null?
    Var(string s){
        sval = s; type = STRING; numericState = UNKNOWN;
    }
    Var(double d){dval = d; type = NUMBER; numericState = NUMERIC;}
    Var(const Var &v){
        sval = string(v.sval); dval = v.dval;
        type = v.type; numericState = v.numericState;
    }
    static bool isNumericString(const string &s) {
        char* ep;
        //cause side-effect: errno can be ERANGE after calling strtod
        strtod(s.c_str(), &ep);
        //Scratch 3.0 recognize the string cause underflows or overflows as Numeric
        return NULL != ep && '\0' == ep[0] && s[0] != '\0';
        // TODO: In Scratch '000' is regarded as non-numeric (but here regarded as numeric)
    }
    bool isNumeric() const{
        if (type == NUMBER) return true;
        if (numericState != UNKNOWN) return numericState == NUMERIC;
        bool numeric = isNumericString(sval);
        numericState = (numeric) ? NUMERIC : STRINGY;
        return numeric;
    }
    double asNumber() const{
        if (type == NUMBER) return dval;
        return (isNumeric()) ? atof(sval.c_str()) : 0.0;
    }
    static bool isNearInteger(const double &x){
        return fabs(round(x) - x) < EPS;
        // TODO: allow integer type in Var class
    }
    static bool isNearNumber(const double &x, const double &y){
        return fabs(x - y) < EPS;
    }
    string asString() const{
        if (type == STRING) return sval;
        if (isNearInteger(dval)) return to_string((ll)round(dval));
        return to_string(dval);
    }
    Var operator+(const Var &y) const{
        return Var(this->asNumber() + y.asNumber());
    }
    Var operator+=(const Var &y){
        *this = *this + y;
        return *this;
    }
    Var operator-(const Var &y) const{
        return Var(this->asNumber() - y.asNumber());
    }
    Var operator*(const Var &y) const{
        return Var(this->asNumber() * y.asNumber());
    }
    Var operator/(const Var &y) const{
        return Var(this->asNumber() / y.asNumber());
    }
    Var operator%(const Var &y) const{
        return Var(fmod(this->asNumber(), y.asNumber()));
    }
    bool operator<(const Var &y) const{
        if (this->isNumeric() && y.isNumeric()){
            return this->asNumber() < y.asNumber();
        }// compare as number if both can be interpreted as numeric
        return this->asString() < y.asString();
    }
    bool operator>(const Var &y) const{
        return y < *this;
    }
    bool operator==(const Var &y) const{
        if (this->isNumeric() && y.isNumeric()){
            return this->asNumber() == y.asNumber();
        }// compare as numeric if both are numeric
        return this->asString() == y.asString();
    }
    friend ostream& operator << (ostream& os, const Var& p);
    friend istream& operator >> (istream& is, const Var& p);
};
ostream& operator << (ostream& os, const Var& p){
    os << p.asString();
    return os;
}
istream& operator >> (istream& is, Var& p){
    string s; is >> s; p = Var(s);
    return is;
}

Var letterOf(Var index, Var sourceString){
    /* index: 1-origined */
    string str = sourceString.asString();
    int idx = (int)(index.asNumber() - 1);
    // seem to be dirty but Scratch seems to do like this.
    // ex. letterOf(0.01, "world") == "w", letterOf(1.99, "world") == "w", letterOf(5.99, "world") == "d"
    if (0 <= idx && idx < str.size()) return Var(str.substr(idx, 1));
    return Var();
}

class VarList{
public:
    vector<Var> data;
    VarList(const vector<Var> &x) { data = x; }
    void push_back(const Var &x){ data.push_back(x); }
    void pop_back(){ data.pop_back(); }
    void clear(){ data.clear(); }
    int size(){ return (int) data.size(); }
    Var getLineOfList(const Var &index) const{
        /* index: 1-origined */
        int idx = (int)index.asNumber() - 1;
        // (unlike 'letterOf', index==0.9 does not work.)
        if (0 <= idx && idx < data.size()) return data[idx];
        return Var();
    }
    void setLineOfListTo(const Var &index, const Var &v){
        /* index: 1-origined */
        int idx = (int)index.asNumber() - 1;
        if (0 <= idx && idx < data.size()) data[idx] = v;
    }
    void deleteLineOfList(const Var &index){
        /* index: 1-origined */
        string kwd = index.asString();
        if (kwd == "all"){
            data.clear();
        }else if (kwd == "last"){
            data.pop_back();
        }else{
            int idx = (int)index.asNumber() - 1;
            if (0 <= idx && idx < data.size()) data.erase(data.begin() + idx);
        }
    }
    void insertAtIndexOfList(const Var &item, const Var &index){
        /* index: 1-origined */
        int idx = (int)index.asNumber() - 1;
        if (0 <= idx && idx <= data.size()) data.insert(data.begin() + idx, item);   
    }
    void insertAtRandomOfList(const Var &item){
        int idx = rand() % (data.size() + 1);
        data.insert(data.begin() + idx, item);
    }
    string asString() const{
        /* concatenate elements of list with space */
        // TODO: concatenated without spaces only if all elements are single characters.
        // (Is it an official bug? (or feature?))
        string ret;
        for(int i=0;i<data.size();i++){
            if (i > 0) ret += ' ';
            ret += data[i].asString();
        }
        return ret;        
    }
    int itemNumOfList(const Var &item) const{
        auto itr = find(data.begin(), data.end(), item);
        if (itr == data.end()) return 0;
        return 1 + (int)(itr - data.begin());
        /* index: 1-origined */
    }
    friend ostream& operator << (ostream& os, const VarList& p);
};
ostream& operator << (ostream& os, const VarList& p){
    os << p.asString();
    return os;
}

double randUniform(double x, double y){
    if (x > y) return randUniform(y, x);
    if (Var::isNearInteger(x) && Var::isNearInteger(y)){
        ll xi = (ll)round(x), yi = (ll)round(y);
        return xi + rand() % (yi - xi + 1);
    }else{
        return x + (y - x) * (0.0 + rand()) / RAND_MAX;
    }
}

Var buf_answer; // for "answer"

// ============================= Scripts =============================

// variable declaration
Var var_H(0);
Var var_W(0);
Var var_h(0);
Var var_w(0);

// list declaration

// prototype declaration of functions
int main();

// contents of functions
int main(){
    cin >> buf_answer;
    var_H = buf_answer;
    cin >> buf_answer;
    var_H = buf_answer;
    cin >> buf_answer;
    var_H = buf_answer;
    cin >> buf_answer;
    var_H = buf_answer;
    cout << ((var_H - var_h) * (var_W - var_w)) << endl;
    return 0;
}