#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, f, k;
  scanf("%d %d %d %d", &a, &b, &f, &k);
  if (b < f) {
    printf("-1\n");
    return 0;
  }
  int cnt = 0;
  int bi = b - f;
  for (int i = 0; i < k - 1; i++) {
    if (i & 1) {
      int dist = (f)*2;
      if (dist > bi) {
        bi = b;
        cnt++;
        if (dist > bi) {
          printf("-1\n");
          return 0;
        }
      }
      bi -= dist;
    } else {
      int dist = (a - f) * 2;
      if (dist > bi) {
        bi = b;
        cnt++;
        if (dist > bi) {
          printf("-1\n");
          return 0;
        }
      }
      bi -= dist;
    }
  }
  if (k & 1) {
    int dist = (a - f);
    if (dist > bi) {
      bi = b;
      cnt++;
      if (dist > bi) {
        printf("-1\n");
        return 0;
      }
    }
  } else {
    int dist = f;
    if (dist > bi) {
      bi = b;
      cnt++;
      if (dist > bi) {
        printf("-1\n");
        return 0;
      }
    }
  }
  printf("%d\n", cnt);
  return 0;
}
