#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <climits>
#include <string>
#include <cmath>
#include <bitset>
#include <complex>
#include <functional>
#include <ctime>
#include <cassert>
#include <fstream>
#include <stack>
#include <random>
#include <iomanip>

#include <fstream>


using namespace std;
typedef long long ll;
typedef long double dd;
#define i_7 (ll)(1E9+7)
//#define i_7 998244353
#define i_5 i_7-2
ll mod(ll a){
    ll c=a%i_7;
    if(c>=0)return c;
    return c+i_7;
}
typedef pair<ll,ll> l_l;
ll inf=(ll)1E18;
#define rep(i,l,r) for(ll i=l;i<=r;i++)
#define pb push_back
ll max(ll a,ll b){if(a<b)return b;else return a;}
ll min(ll a,ll b){if(a>b)return b;else return a;}
dd EPS=1E-9;
#define endl "\n"
#define fastio ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);



int main(){fastio
    ll a[4];rep(i,0,3)cin>>a[i];
    cout<<a[0]*a[3]-a[1]*a[2]<<endl;
    
    return 0;
}




/*
これを入れて実行
g++ code.cpp
./a.out
 */
 
#include <iostream>
#include <cstdio>
#include <stdio.h>
#include <vector>
#include <string>
#include <cstring>
#include <queue>
#include <deque>
#include <stack>
#include <algorithm>
#include <utility>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <cmath>
#include <math.h>
#include <tuple>
#include <iomanip>
#include <bitset>
#include <functional>
#include <cassert>
#include <random>

#define all(x) (x).begin(),(x).end()
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
 
using namespace std;
typedef long long ll;
typedef long double ld;
 
int dy4[4] = {-1, 0, +1, 0};
int dx4[4] = {0, +1, 0, -1};
int dy8[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
int dx8[8] = {0, 1, 1, 1, 0, -1, -1, -1};
 
const long long INF = 1LL << 61;
const ll MOD = 1e9 + 7;
 
bool greaterSecond(const pair<int, int>& f, const pair<int, int>& s){
    return f.second > s.second;
}
 
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 conbinationMemo[201][12];

void cmemoInit(){
    rep(i, 201){
        rep(j, 12){
            conbinationMemo[i][j] = -1;
        }
    }
}
 
ll nCr(ll n, ll r){
    if(conbinationMemo[n][r] != -1) return conbinationMemo[n][r];
    if(r == 0 || r == n){
        return 1;
    } else if(r == 1){
        return n;
    }
    return conbinationMemo[n][r] = (nCr(n - 1, r) + nCr(n - 1, r - 1));
}
 
ll nPr(ll n, ll r){
    r = n - r;
    ll ret = 1;
    for (ll i = n; i >= r + 1; i--) ret *= i;
    return ret;
}
//-----------------------ここから-----------
int main(void){
    ll n;
    cin >> n;
    cout << n - 1 << endl;
}