#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
const int INF = 0x3f3f3f3f;
const long long LL_INF = 0x3f3f3f3f3f3f3f3f;
const double PI = acos(-1);
const double EPS = 1e-10;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int a, b, f, k;
  cin >> a >> b >> f >> k;
  if (b < f) {
    cout << -1;
    return 0;
  }
  int cur = b - f, res = 0;
  for (int i = 1; i < k; ++i) {
    int toDest = i & 1 ? a - f : f;
    int backToStation = toDest << 1;
    if (backToStation > cur) {
      if (backToStation > b) {
        cout << -1;
        return 0;
      }
      ++res;
      cur = b;
    }
    cur -= backToStation;
  }
  int toDest = k & 1 ? a - f : f;
  if (toDest > cur) {
    if (toDest > b) {
      cout << -1;
      return 0;
    }
    ++res;
    cur = b;
  }
  cur -= toDest;
  cout << res;
}
