l=[int(x) for x in input().split()]
a=[]
b=l[1]
f=0
c=0
a.append(0)
for i in range(l[3]):
if(i%2==0):
a.append(i*l[0]+l[2])
else:
a.append((i+1)*l[0]-l[2])
a.append(l[0]*l[3])
for j in range(1,l[3]+2):
if(b>=(a[j]-a[j-1])):
b=b-a[j]
b=b+a[j-1]
else:
b=l[1]
if(b>=a[j]-a[j-1]):
f=f+1
b=b-(a[j]-a[j-1])
else:
c=-1
break
if(c==-1):
print(c)
else:
print(f)#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int a,b,f,k;
int main(){
cin>>a>>b>>f>>k;
int t=0,b1=b,i=0,ans=0;
while(k--){
if(i==0){
t=f;
b1=b1-f;
if(b1<0){
cout<<-1;
return 0;
}
if(b1<2*(a-f)&&k!=0){
b1=b;
ans++;
}
else if(k==0&&b1<a-f){
b1=b;
ans++;
}
t=a;
b1=b1-(a-f);
if(b1<0){
cout<<-1;
return 0;
}
i=1;
}
else if(i==1){
t=f;
b1=b1-(a-f);
if(b1<0){
cout<<-1;
return 0;
}
if(b1<2*f&&k!=0){
b1=b;
ans++;
}
else if(k==0&&b1<f){
b1=b;
ans++;
}
t=0;
b1=b1-f;
if(b1<0){
cout<<-1;
return 0;
}
i=0;
}
// cout<<b1<<" "<<ans<<endl;
}
cout<<ans;
return 0;
}
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