#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e4 + 10;
int stops[maxn];
int x = 0, a, b, f, k, c, dist = 0, curr;
int main() {
  scanf("%d%d%d%d", &a, &b, &f, &k);
  int l = f;
  int r = a - f;
  if (k != 0) stops[0] = l;
  for (int i = 1; i < k; i++) {
    if (i & 1)
      stops[i] = 2 * r;
    else
      stops[i] = 2 * l;
  }
  if (k & 1)
    stops[k] = r;
  else
    stops[k] = l;
  for (int i = 0; i <= k; i++) curr = b;
  for (int i = 0; i <= k; i++) {
    if (curr >= stops[i]) {
      curr -= stops[i];
      if (stops[i + 1] > curr) {
        x++;
        curr = b;
      }
    } else if (curr < stops[i]) {
      x = -1;
      break;
    }
  }
  cout << x << endl;
  return 0;
}
