#include <bits/stdc++.h>
using namespace std;
int main() {
  int t;
  long long c, d, l, n, a, b, f, k;
  long long min, max;
  string x, y;
  cin >> a >> b >> f >> k;
  long long curr = b;
  long long count = 0;
  c = f > a - f ? f : a - f;
  if (k == 1) {
    if (b >= a)
      cout << 0;
    else if (b >= f && b + f >= a) {
      cout << 1;
    } else
      cout << -1;
  } else if (b >= 2 * c) {
    for (int i = 0; i < k; i++) {
      if (i == 0) {
        if (curr < (2 * a - f)) {
          curr = b - 2 * (a - f);
          count++;
        } else
          curr -= 2 * a - f;
        continue;
      } else if (i == (k - 1) && k % 2 == 0) {
        if (curr >= f)
          continue;
        else {
          curr = b - f;
          count++;
        }
        continue;
      } else if (i == k - 1) {
        if (curr >= (a - f))
          continue;
        else {
          curr = b - (a - f);
          count++;
        }
        continue;
      }
      if (i % 2 == 0) {
        if (curr >= 2 * (a - f))
          curr -= 2 * (a - f);
        else {
          curr = b - 2 * (a - f);
          count++;
        }
      } else {
        if (curr >= 2 * (f))
          curr -= 2 * (f);
        else {
          curr = b - 2 * (f);
          count++;
        }
      }
    }
    cout << count;
  } else if (k == 2 && b >= f && b >= 2 * (a - f)) {
    if (b >= (2 * a - f))
      cout << 1;
    else if (b >= 2 * (a - f))
      cout << 2;
  } else
    cout << -1;
}
