#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long a, b, k, f;
  cin >> a >> b >> f >> k;
  long long flag = 0, cnt = 0;
  long long p = b;
  if (k == 1) {
    if (p >= a) {
      cout << "0\n";
    } else if (p >= f && b >= (a - f)) {
      cout << "1\n";
    } else
      cout << "-1\n";
    return;
  }
  for (long long i = 1; i <= k; i++) {
    if (i == k) {
      if (p >= a)
        ;
      else if (p >= f && b >= (a - f)) {
        cnt++;
      } else {
        flag = 1;
        break;
      }
      break;
    }
    if (p < f) {
      flag = 1;
      break;
    }
    if (2 * (a - f) <= (p - f)) {
      p -= f;
      p -= 2 * (a - f);
    } else {
      p = b;
      cnt++;
      if (2 * (a - f) <= (p)) {
        p -= 2 * (a - f);
      } else {
        flag = 1;
        break;
      }
    }
    i++;
    if (i > k) break;
    if (i == k) {
      if (p >= f)
        ;
      else if (b >= f) {
        cnt++;
      } else {
        flag = 1;
        break;
      }
    } else {
      if (p >= 2 * (f)) {
        p -= f;
      } else {
        if (b >= (2 * f)) {
          cnt++;
          p = b - f;
        } else {
          flag = 1;
          break;
        }
      }
    }
  }
  if (flag) {
    cout << "-1\n";
    return;
  }
  cout << cnt << "\n";
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long test = 0, t;
  t = 1;
  while (test++ < t) {
    solve();
  }
}
