#include <bits/stdc++.h>
using namespace std;
int main() {
  int x_end, stop, max_l, k;
  int l, ans = 0, end, dist, f = 2;
  scanf("%d %d %d %d", &x_end, &max_l, &stop, &k);
  l = max_l;
  for (int i = 1; i <= k; i++) {
    if (i & 1)
      end = 0;
    else
      end = x_end;
    dist = abs(end - stop);
    l -= dist;
    if (l < 0) {
      ans = -1;
      break;
    }
    if (k == i) f = 1;
    if (l < f * (x_end - dist)) {
      ans++;
      l = max_l;
      if (l < f * (x_end - dist)) {
        ans = -1;
        break;
      }
    }
    l -= x_end - dist;
  }
  printf("%d\n", ans);
  return 0;
}
