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