#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
char IO;
int rd(){
	int num=0;bool f=0;
	while(IO=getchar(),IO<48||IO>57)if(IO=='-')f=1;
	do num=(num<<1)+(num<<3)+(IO^48);
	while(IO=getchar(),IO>=48&&IO<=57);
	return f?-num:num;
}
ll A,B,C,D;
int main(){
	A=rd(),B=rd(),C=rd(),D=rd();
	if(C*D<=B){cout<<-1;return 0;}
	cout<<(A-1)/(C*D-B)+1;
	return 0;
}

 #include <bits/stdc++.h>
 using namespace std;
 int main() {
   int A,B,C,D;
   cin>>A>>B>>C>>D;
   cout<<A*D-B*C<<endl;
 }
