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