#include <bits/stdc++.h>
using namespace std;
int main() {
  int lst, fuel, gs, jt;
  cin >> lst >> fuel >> gs >> jt;
  int Ans = 0;
  bool ck = false;
  if (fuel - gs < 0) ck = true;
  int stock = fuel - gs;
  int tmp;
  for (int i = 1; i <= jt and !ck; i++) {
    if (i & 1) {
      int dif = lst - gs;
      if (i == jt)
        tmp = dif;
      else
        tmp = 2 * dif;
    } else {
      if (i == jt)
        tmp = gs;
      else
        tmp = 2 * gs;
    }
    if (tmp > stock) {
      Ans++;
      stock = fuel;
    }
    stock -= tmp;
    if (stock < 0) {
      ck = true;
      break;
    }
  }
  if (ck)
    cout << -ck << endl;
  else
    cout << Ans << endl;
  return 0;
}
