#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e3 + 5;
const long long mod = 1e9 + 7;
int main() {
  int a, b, f, k;
  cin >> a >> b >> f >> k;
  int ans = 0;
  if (k == 1 && (b < f || b < (a - f))) {
    printf("-1\n");
    return 0;
  } else if (k == 2 && (b < 2 * (a - f) || b < f)) {
    printf("-1\n");
    return 0;
  }
  if (k >= 3 && (b < 2 * (a - f) || b < 2 * f)) {
    printf("-1\n");
    return 0;
  }
  int l1 = 2 * (a - f), l2 = 2 * f;
  int b1 = b - f;
  for (int i = 1; i < k; i++) {
    if (i % 2 == 1) {
      if (b1 >= l1)
        b1 -= l1;
      else {
        b1 = b;
        b1 -= l1;
        ans++;
      }
    } else {
      if (b1 >= l2)
        b1 -= l2;
      else {
        b1 = b;
        b1 -= l2;
        ans++;
      }
    }
  }
  if (k % 2 == 1) {
    if (b1 < (a - f)) ans++;
  } else {
    if (b1 < f) ans++;
  }
  cout << ans << endl;
  return 0;
}
