#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, f, k;
  vector<int> v;
  long long sum = 0;
  cin >> a >> b >> f >> k;
  k--;
  v.push_back(f);
  int i;
  for (i = 0; i < k; i++) {
    if (i % 2 == 0)
      v.push_back(2 * (a - f));
    else
      v.push_back(2 * f);
  }
  if (i % 2 == 0)
    v.push_back(a - f);
  else
    v.push_back(f);
  int ans = 0;
  for (i = 0; i < v.size(); ++i) {
    sum += (long long)v[i];
    if (sum > b) {
      ans++;
      sum = v[i];
      if (sum > b) {
        cout << "-1";
        return 0;
      }
    }
  }
  cout << ans;
  return 0;
}
