C

#include<bits/stdc++.h>
#define pii pair<int, int>
#define endl '\n'
#define pb push_back
#define lli long long int
#define mod 1000000007

using namespace std;
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    bool chk=true;
    lli fin, tank, gas, cnt, time=0, now, i;
    cin >> fin >> tank >> gas >> cnt;now=tank;
    for(i=1;i<=cnt;i++)
    {
        if(i==cnt && now>=fin) continue;
        if(chk)
        {
            if(now < gas) cout<<"-1", exit(0);
            if(fin+(fin-gas) <= now) now-=fin;
            else now=tank-(fin-gas), time++;
            chk=false;
        }
        else
        {
            if(now < fin-gas) cout<<"-1", exit(0);
            if(fin+gas <= now) now-=fin;
            else now=tank-gas, time++;
            chk=true;
        }
    }
    cout<<time<<endl;
    return 0;
}