#include<bits/stdc++.h>
using namespace std;
template <typename T>
inline void read(T &x){
    x=0;char ch=getchar();bool f=false;
    while(!isdigit(ch)) f|=ch=='-',ch=getchar();
    while(isdigit(ch))  x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    x=f?-x:x;
}
template <typename T>
inline void print(T x){
    if(x<0) putchar('-'),x=-x;
    if(x>9) print(x/10);
    putchar(x%10^48);
}
int x,y,z;
 
int main(){
    read(x),read(y),read(z);
    for(register int i=0;i<=1000000;++i){
        double tep=double((1.0*i)/z),tep2=double((1.0*y)/x);
        if(tep>=tep2){
            // cout<<tep<<" "<<tep2<<endl;
            print(i-1);break;
        }
    }
    return 0;
}
#include<iostream>             
#include<vector>   
#include<bits/stdc++.h>
#define mod 1000000007
#define pi 3.14159265358979
#define  PRE(x,p) cout<<setprecision(x)<<p; 
#define pb push_back
#define mp make_pair
#define f first
#define s second
#define endl "\n"
#define high INT_MAX
#define low INT_MIN
#define ll long long int

using namespace std;


void showa(ll a[],ll n){ for(ll i=1;i<=n;i++) cout<<a[i]<<' ';  cout<<endl;  }
void showv(vector<ll>v){ for(ll i=0;i<v.size();i++) cout<<v[i]<<' '; cout<<endl;}


void solve() 
{ 
   ll n1,b,c,d,i;
   cin>>n1>>b>>c>>d;
   ll n2=0;
   
   for(i=1;i<=1000000;i++)
   {
    if(n1<=n2*d) 
    {cout<<i-1;
    return;}

    n1+=b;
    n2+=c;
   }

   cout<<"-1";

}
int main()
{
 #ifndef ONLINE_JUDGE
 freopen("input.txt", "r", stdin);
 freopen("output.txt", "w", stdout);
 #endif
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);

ll t=1;
// cin>>t;
for(ll test=1;test<=t;test++)
{
    // cout<<"CASE #"<<test<<": ";
    solve();
}

return 0;
}