#include <bits/stdc++.h>
using namespace std;
void solve();
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed;
  cout.precision(12);
  solve();
  return 0;
}
template <typename T>
void sc(T& x) {
  cin >> x;
}
template <typename Head, typename... Tail>
void sc(Head& head, Tail&... tail) {
  cin >> head;
  sc(tail...);
}
template <typename T>
void pr(const T& x) {
  cout << x << '\n';
}
template <typename Head, typename... Tail>
void pr(const Head& head, const Tail&... tail) {
  cout << head << ' ';
  pr(tail...);
}
void solve() {
  int a, b, f, k;
  sc(a, b, f, k);
  if (b < f) {
    pr(-1);
    return;
  }
  int dir = 1, cap = b - f, ans = 0;
  while (k--) {
    int dist = (dir ? a - f : f);
    if (k > 0) {
      dist *= 2;
    }
    if (b < dist) {
      pr(-1);
      return;
    }
    if (cap < dist) {
      cap = b;
      ans++;
    }
    cap -= dist;
    dir ^= 1;
  }
  pr(ans);
}
