#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, f, k, need[2];
  scanf("%d%d%d%d", &a, &b, &f, &k);
  need[1] = (a - f) << 1, need[0] = f << 1;
  if (b < f || b < a - f) return 0 * puts("-1");
  int now = b - f, ans = 0;
  for (int t = 1; t < k; t++) {
    if (now < need[t & 1]) now = b, ans++;
    now -= need[t & 1];
    if (now < 0) return 0 * puts("-1");
  }
  if (now < need[k & 1] / 2) ans++;
  printf("%d\n", ans);
  return 0;
}
