#include<stdio.h>#include<string.h>#include<math.h>#include<stdlib.h>#include<algorithm>#include<map>#include<vector>#include<iostream>using namespace std;int main(){ double n,k,l,m; long long sum,ss; while(cin>>n>>k>>l>>m) { if(k<=l) {puts("0");continue;} double sta=k+0.0499999999999; sta=min(sta,n*1.0); l+=0.0499999999999; l=min(n*1.0,l); long long low,mid,high,tmp; sum=sta*m; sum=(long long)sum; low=0;ss=tmp=high=0x7f7f7f7f7f7f7fll; while(low<=high) { mid=(low+high)>>1; tmp=(long long)(l*(mid+m)); if((sum+mid)<=tmp) high=mid-1; else low=mid+1; } if(low>ss) { cout<<"-1"<<endl;continue; } cout<<low<<endl; }}
