#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  long long int a, b, f, k;
  cin >> a >> b >> f >> k;
  long long int ans = 0, currf = b - f, currp = 0, tdis;
  long long int i;
  if (b < f) {
    cout << -1 << '\n';
    return 0;
  }
  for (i = 1; i <= k; i++) {
    if (i == k) {
      if (i % 2)
        tdis = (a - f);
      else
        tdis = f;
      if (currf < tdis) {
        if (b < tdis) {
          ans = -1;
          break;
        } else {
          ans++;
          currf = b;
          -tdis;
        }
      } else {
        currf -= tdis;
      }
    } else {
      if (i % 2)
        tdis = 2 * (a - f);
      else
        tdis = 2 * f;
      if (currf < tdis) {
        if (b < tdis) {
          ans = -1;
          break;
        } else {
          ans++;
          currf = b - tdis;
        }
      } else {
        currf -= tdis;
      }
    }
  }
  cout << ans << '\n';
  return 0;
}
