#include <bits/stdc++.h>
using namespace std;
long long int arr[10004];
int main() {
  long long int a, b, f, k, i, temp, x;
  cin >> a >> b >> f >> k;
  arr[0] = f;
  for (i = 1; i < k; i++) {
    if (i % 2 == 0) {
      arr[i] = i * a + f;
    } else {
      arr[i] = (i + 1) * a - f;
    }
  }
  if (arr[k - 1] < k * a) {
    arr[k] = k * a;
    k++;
  }
  long long int counter = 0;
  temp = b;
  int flag = 0, last = -1;
  while (1) {
    x = lower_bound(arr, arr + k, b) - arr;
    if (arr[x] > b) {
      if (x == 0) {
        flag = 1;
        break;
      }
      x--;
    }
    if (last == x) {
      flag = 1;
      break;
    }
    last = x;
    if (x >= k)
      break;
    else {
      if (x != k - 1) counter++;
      b = temp + arr[x];
    }
  }
  if (flag == 1) {
    cout << "-1\n";
  } else {
    cout << counter << endl;
  }
}
