#include <bits/stdc++.h>
using namespace std;
int n, m, k;
int x[105];
int main() {
  long long a, b, f, k;
  cin >> a >> b >> f >> k;
  if (((f * 2 > b || (a - f) * 2 > b) && k > 2) ||
      (k == 1 && (f > b || a - f > b)) ||
      (k == 2 && (f > b || (a - f) * 2 > b))) {
    cout << -1;
    return 0;
  }
  long long ans = 0, cur = b;
  vector<long long> p;
  for (long long i = 1; i <= k; i++) {
    if (i % 2 == 1) {
      p.push_back((i - 1LL) * a + f);
    } else {
      p.push_back(i * a - f);
    }
  }
  p.push_back(a * k);
  for (int i = 0; i < p.size(); i++) {
    if (p[i] > cur) {
      ans++;
      cur = b + p[i - 1];
    }
  }
  cout << ans;
  return 0;
}
