#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, f, k, count = 0, b1;
  cin >> a >> b >> f >> k;
  b1 = b;
  for (int i = 1; i <= k; i++) {
    if (i % 2 == 1) {
      if (b < f) {
        cout << "-1";
        return 0;
      } else {
        b -= f;
        if (b < a - f) {
          count++;
          b = b1;
          if (b < a - f) {
            cout << "-1";
            return 0;
          } else {
            b = b - a + f;
          }
        } else {
          b = b - a + f;
          if (b < a - f && i != k) {
            count++;
            b = b1;
            b = b - a + f;
          }
        }
      }
    } else {
      if (b < a - f) {
        cout << "-1";
        return 0;
      } else {
        b = b - a + f;
      }
      if (b < f) {
        count++;
        b = b1;
        if (b < f) {
          cout << "-1";
          return 0;
        } else {
          b -= f;
        }
      } else {
        b -= f;
        if (b < f && i != k) {
          count++;
          b = b1;
          b = b - f;
        }
      }
    }
  }
  cout << count;
  return 0;
}
