#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:102400000,102400000")
using namespace std;
int main() {
  long long a, b, f, k;
  scanf("%lld%lld%lld%lld", &a, &b, &f, &k);
  if (k == 1) {
    if (b >= a)
      printf("0\n");
    else if (b >= f && b >= a - f)
      printf("1\n");
    else
      printf("-1\n");
    return 0;
  }
  if (k == 2) {
    if (b < f || b < 2 * (a - f))
      printf("-1\n");
    else if (b >= 2 * a)
      printf("0\n");
    else if (b >= a + (a - f))
      printf("1\n");
    else
      printf("2\n");
    return 0;
  }
  if (f * 2 > b || ((a - f) * 2) > b) {
    printf("-1\n");
    return 0;
  }
  long long cur = b;
  int cnt = 0;
  for (int i = (1); i < k + 1; i++) {
    if (i == k) {
      if (i & 0x1) {
        cur -= f;
        if (cur < (a - f)) cur = b, cnt++;
        cur -= a - f;
      } else {
        cur -= a - f;
        if (cur < (f)) cur = b, cnt++;
        cur -= f;
      }
      break;
    }
    if (i & 0x1) {
      cur -= f;
      if (cur < 2 * (a - f)) cur = b, cnt++;
      cur -= a - f;
    } else {
      cur -= a - f;
      if (cur < 2 * (f)) cur = b, cnt++;
      cur -= f;
    }
  }
  printf("%d", cnt);
  return 0;
}
