#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int a, b, f, k;
  cin >> a >> b >> f >> k;
  long long int ans = 0;
  int nowB = b;
  for (int i = 1; i <= k + 1; i++) {
    bool isRight = ((i == 1) || (i % 2 == 0));
    int run;
    if (i == 1) {
      run = f;
    } else if (i == k + 1) {
      if (isRight) {
        run = a - f;
      } else {
        run = f;
      }
    } else {
      if (isRight) {
        run = 2 * (a - f);
      } else {
        run = 2 * f;
      }
    }
    if (run > b) {
      cout << "-1\n";
      return 0;
    }
    if (run <= nowB) {
      nowB -= run;
    } else {
      ans++;
      nowB = b - run;
    }
  }
  cout << ans << "\n";
  return 0;
}
