#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int a, c, b, k, i, p, j, count = 0, n, m, l = 1;
  cin >> a >> c >> b >> k;
  n = 2 * b;
  m = 2 * (a - b);
  if ((k == 1) && (c < b || c < a - b)) {
    count = -1;
  } else if ((k == 2) && (c < b || (c < m))) {
    count = -1;
  } else if (k > 2) {
    if (c < n || c < m) count = -1;
  }
  if (count != -1) {
    p = c - b;
    for (i = 0; i < k - 1; i++) {
      if (i % 2 == 0) {
        l = 0;
        if (p < m) {
          p = c;
          count++;
        }
        p = p - m;
      } else {
        l = 1;
        if (p < n) {
          p = c;
          count++;
        }
        p = p - n;
      }
    }
    if (l == 0) {
      if (p < b) {
        count++;
      }
    } else if (l == 1) {
      if (p < a - b) {
        count++;
      }
    }
  }
  cout << count;
  return 0;
}
