#include <bits/stdc++.h>
int main() {
  int a, b, f, k;
  while (~scanf("%d%d%d%d", &a, &b, &f, &k)) {
    if ((b < f) || (b < a - f)) {
      printf("-1\n");
      continue;
    } else if (k == 2 && b < 2 * (a - f)) {
      printf("-1\n");
      continue;
    } else if (k > 2 && b < a) {
      printf("-1\n");
      continue;
    }
    int bag = b;
    int flag1 = 1, flag2 = 0, count = 0;
    for (int i = 1; i <= k; i++) {
      if (flag1) {
        bag -= f;
        if ((i != k && bag < 2 * (a - f)) || (i == k && bag < a - f)) {
          bag = b;
          count++;
        }
        bag -= (a - f);
        flag1 = 0;
        flag2 = 1;
      } else if (flag2) {
        bag -= (a - f);
        if ((i != k && bag < 2 * f) || (i == k && bag < f)) {
          bag = b;
          count++;
        }
        bag -= f;
        flag2 = 0;
        flag1 = 1;
      }
    }
    printf("%d\n", count);
  }
  return 0;
}
