#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, f, k;
  scanf("%lld%lld%lld%lld", &a, &b, &f, &k);
  long long left = f * 2;
  long long right = (a - f) * 2;
  if (b < f || b < (a - f)) {
    printf("-1");
    return 0;
  }
  long long tank = b;
  long long x = 1, ans = 0;
  long long A[2];
  A[0] = left;
  A[1] = right;
  tank -= f;
  for (int i = 1; i < k; i++) {
    if (b < A[x]) {
      printf("-1");
      return 0;
    }
    if (tank < A[x]) {
      ans++;
      tank = b;
    }
    tank -= A[x];
    x = !x;
  }
  if (tank < A[x] / 2) ans++;
  printf("%lld", ans);
  return 0;
}
