#include <bits/stdc++.h>
using namespace std;
int a, b, f, k;
int main() {
  scanf("%d%d%d%d", &a, &b, &f, &k);
  if (((a - f) * 2 > b && k > 1) || (k == 1 && a - f > b) ||
      (k > 2 && f * 2 > b) || (k == 1 && f > b)) {
    printf("-1");
    return 0;
  }
  int ff;
  int kk = -1;
  int v = 1;
  int q = f, h = a - f;
  int ans = 0, o = b;
  while (kk < k) {
    if (v == 1) {
      kk++;
      v = 2;
      o -= q;
    } else if (v == 2) {
      if (2 * h > o) {
        if (kk + 1 == k && h <= o) {
          ff = 0;
        } else {
          o = b;
          ans++;
        }
      }
      o -= h;
      v = 3;
    } else if (v == 3) {
      kk++;
      v = 4;
      o -= h;
    } else {
      if (2 * q > o) {
        if (kk + 1 == k && q <= o) {
          ff = 0;
        } else {
          o = b;
          ans++;
        }
      }
      o -= q;
      v = 1;
    }
  }
  printf("%d", ans);
  return 0;
}
