#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, t, f, k, y = 0, w, j = 0, x = 0;
  cin >> a >> t >> f >> k;
  w = t;
  for (;;) {
    if (x == 0) {
      if (j % 2 == 1) j++;
      if (j == k) break;
      if (w >= f) {
        x = f;
        w -= f;
      } else {
        cout << -1;
        return 0;
      }
    } else if (x == f) {
      if (j % 2 == 0) {
        if (w >= (a - f) * 2) {
          x = a;
          w -= (a - f);
        } else if (w >= a - f) {
          if (j == k - 1) {
            x = a;
            w -= (a - f);
          } else {
            w = t;
            x = a;
            w -= (a - f);
            y++;
          }
        } else {
          if (t >= a - f) {
            w = t;
            x = a;
            w -= (a - f);
            y++;
          } else {
            cout << -1;
            return 0;
          }
        }
      } else {
        if (w >= f * 2) {
          x = 0;
          w -= f;
        } else if (w >= f) {
          if (j == k - 1) {
            x = 0;
            w -= f;
          } else {
            w = t;
            x = 0;
            w -= f;
            y++;
          }
        } else {
          if (t >= f) {
            w = t;
            x = 0;
            w -= f;
            y++;
          } else {
            cout << -1;
            return 0;
          }
        }
      }
    } else if (x == a) {
      if (j % 2 == 0) j++;
      if (j == k) break;
      if (w >= a - f) {
        x = f;
        w -= (a - f);
      } else {
        cout << -1;
        return 0;
      }
    }
    if (j == k) break;
  }
  cout << y;
}
