#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
void yes() {
  cout << "Yes"
       << "\n";
}
void no() {
  cout << "No"
       << "\n";
}
struct Sync_stdio {
  Sync_stdio() {
    cin.tie(NULL);
    ios_base::sync_with_stdio(false);
  }
} _sync_stdio;
struct FAIL {
  FAIL() {
    cout << "CHANGE!!!"
         << "\n";
  }
};
int main() {
  long long a, b, f, k;
  cin >> a >> b >> f >> k;
  int current = b;
  int res = 0;
  for (int i = (0); i < (k + 1); ++i) {
    int need_dist = 0;
    if (i == 0) {
      need_dist = f;
    } else if (i == k) {
      need_dist = i % 2 ? a - f : f;
    } else if (i % 2) {
      need_dist = 2 * (a - f);
    } else {
      need_dist = 2 * f;
    }
    if (current < need_dist) {
      ++res;
      current = b;
    }
    if (b < need_dist) {
      cout << -1;
      return 0;
    }
    current -= need_dist;
  }
  cout << res;
  return 0;
}
