#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define pob pop_back
void solve()
{
    int a,b,c,d;
    cin>>a>>b>>c>>d;
    cout<<(a*d)-(b*c);
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    // #ifndef ONLINE_JUDGE
    //     freopen("input.txt","r+",stdin);
    //     freopen("output.txt","w+",stdout);
    // #endif
    solve();
}

// International Master sm0016
#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define all(x) x.begin(),x.end()
#define raftaar ios_base::sync_with_stdio(false);cin.tie(NULL);
using namespace std;
const ll MOD1=1000000007;

const ll MOD2= 998244353;

const int N=1e6+6;


int main(){
	raftaar
	
	int x,y,z;cin>>x>>y>>z;int ans;

	int zz=0;

	while(y*z>x*zz){

		zz++;
	}

	cout<<zz-1;
}
