#include <bits/stdc++.h>
using namespace std;
int main() {
  long a, b, f, k, i, full, count = 0;
  cin >> a >> b >> f >> k;
  full = b;
  if (b < f || b < (a - f))
    count = -1;
  else {
    for (i = 0; i < k; i++) {
      if (i % 2 == 0) {
        if (b < f) {
          count = -1;
          break;
        }
        if (i == k - 1) {
          if (b < a) count++;
        } else if (b >= (2 * a - f)) {
          b = b - a;
        } else {
          b = full;
          b = b - (a - f);
          count++;
        }
      }
      if (i % 2 == 1) {
        if (b < a - f) {
          count = -1;
          break;
        }
        if (i == k - 1) {
          if (b < a) count++;
        } else if (b >= a + f) {
          b = b - a;
        } else {
          b = full;
          b = b - f;
          count++;
        }
      }
    }
  }
  cout << count;
  return 0;
}
