#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e6 + 5;
int dp[maxn];
int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  if (d == 1) {
    if (b >= a)
      cout << 0;
    else if (c <= b && (a - c) <= b)
      cout << 1;
    else
      cout << -1;
  } else if (d == 2) {
    if (b < 2 * (a - c) || b < c)
      cout << -1;
    else if (b < 2 * (a - c) + c)
      cout << 2;
    else if (b >= 2 * a)
      cout << 0;
    else
      cout << 1;
  } else {
    int ans = 0, now = b - c;
    long long need = 1ll * a * d, g = c;
    if (b < 2 * c || b < 2 * (a - c)) {
      cout << -1;
      return 0;
    }
    for (int i = 1; i <= d; i++) {
      int next;
      if (i % 2 == 1)
        next = 2 * (a - c);
      else
        next = 2 * c;
      if (now >= need - g) {
        break;
      } else if (now >= next)
        now -= next;
      else {
        now = b - next;
        ans++;
      }
      g += next;
    }
    cout << ans;
  }
}
