#include <map>
#include <set>
#include <list>
#include <cmath>
#include <deque>
#include <stack>
#include <queue>
#include <array>
#include <bitset>
#include <cstdio>
#include <string>
#include <vector>
#include <random>
#include <chrono>
#include <utility>
#include <numeric>
#include <cstdlib>
#include <cstring>
#include <climits>
#include <sstream>
#include <assert.h>
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <unordered_map>
using namespace std;

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

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...);}
#ifdef HOME
#warning CHECK int:ll::INT_MAX:LLONG_MAX
#define maxn 20
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define maxn 2000006
#define gcd __gcd
#define debug(x...)
#endif

#define ff first
#define endl '\n'
#define ss second
#define inf 0x3f3f3f3f
#define MOD 1000000007
#define PI 3.14159265358979323846264338327950L
#define f(i,x,n) for(int i=x;i<=n;i++)
#define fr(i,x,n) for(int i=x;i>=n;i--)
struct _ { ios_base::Init i; _() { ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL); } } _;

int dx[] = { -1, 0, 1, 0, -1, -1, 1, 1};
int dy[] = {0, 1, 0, -1, -1, 1, 1, -1};


int main() {

	int a, b, c, d; cin >> a >> b >> c >> d;
	cout << a*d - b*c;

	return 0;
}
// ConsoleApplication1.cpp : Defines the entry point for the console application.
//

#include <stack>
#include <iostream>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <cmath>
#include <queue>

#define MAXCHAR 255
#define ll long long

using namespace std;
const ll dividend = 1000000007;
const ll limit = pow(10, 18);
const ll bigNum = pow(10, 9);

ll funcComb(int n, int k)
{
	ll output = 1;
	int sum = n + k;
	vector<int> dividends;
	for (int i = 0; i < k; i++)
	{
		dividends.push_back(k - i);
	}
	for (int i = 0; i < k; i++)
	{
		output *= sum - i;
		for (auto it = dividends.begin(); it != dividends.end(); it++)
		{
			if (*it == 0)
				continue;
			if (output % *it == 0)
			{
				output /= *it;
				*it = 0;
			}
		}
		if (output > dividend)
		{
			output %= dividend;
		}
	}

	return output;
}

ll funcPow(int n, int r)
{
	ll ret = 1;
	for (int i = 1; i <= r; i++)
	{
		ret *= n;
		if (ret > dividend)
			ret %= dividend;
	}
	return ret;
}

vector<int> broken;

ll funcA(int step)
{
	for (auto it = broken.begin(); it != broken.end(); it++)
	{
		if (step == *it)
			return 0;
	}
	if (step == 1)
		return 1;
	if (step == 2)
		return 2;
	return funcA(step - 1) % dividend + funcA(step - 2) % dividend;
}

vector<int> C;
vector<vector<int>> tree;

int main()
{
	int X, Y, Z, out;
	cin >> X >> Y >> Z;

	out = Z * Y / X;
	if (out * X == Z * Y)
		out--;

	cout << out << endl;
	return 0;
}

