#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, bb, f, k, j = 0, c = 0;
  bool r = 0;
  cin >> a >> b >> f >> k;
  bb = b;
  if (b < f) {
    cout << -1;
    return 0;
  }
  b -= f;
  while (j < k - 1) {
    if (!r) {
      if ((a - f) * 2 > b && (a - f) * 2 <= bb) {
        b = bb;
        j++;
        b -= (a - f) * 2;
        r = 1;
        c++;
      } else if ((a - f) * 2 <= b) {
        b -= (a - f) * 2;
        r = 1;
        j++;
      } else if ((a - f) * 2 > bb) {
        cout << -1;
        return 0;
      }
    } else {
      if (f * 2 > b && f * 2 <= bb) {
        b = bb;
        j++;
        b -= f * 2;
        r = 0;
        c++;
      } else if (f * 2 <= b) {
        b -= f * 2;
        r = 0;
        j++;
      } else {
        cout << -1;
        return 0;
      }
    }
  }
  if (!r) {
    if ((a - f) > b && (a - f) <= bb) {
      b = bb;
      j++;
      b -= (a - f);
      c++;
    } else if ((a - f) <= b) {
      b -= (a - f);
      r = 1;
      j++;
    } else if ((a - f) > bb) {
      cout << -1;
      return 0;
    }
  } else {
    if (f > b && f <= bb) {
      b = bb;
      j++;
      b -= f;
      r = 0;
      c++;
    } else if (f <= b) {
      b -= f;
      r = 0;
    } else {
      cout << -1;
      return 0;
    }
  }
  cout << c;
  return 0;
}
