#include <bits/stdc++.h>
using namespace std;
vector<long long> v;
long long a, b, f, k;
int cnt() {
  int c = 0;
  int n = v.size();
  long long now = 0;
  for (int i = 0; i < n; i++) {
    if (now + b < v[i]) return -1;
    while (i < n && now + b >= v[i]) i++;
    i--;
    c++;
    now = v[i];
  }
  return c - 1;
}
int main() {
  scanf("%I64d%I64d%I64d%I64d", &a, &b, &f, &k);
  v.push_back(f);
  for (long long i = 1; i < k; i++) {
    if (i & 1)
      v.push_back(v[i - 1] + 2 * (a - f));
    else
      v.push_back(v[i - 1] + 2 * f);
  }
  v.push_back(a * k);
  printf("%d", cnt());
}
