#include <bits/stdc++.h>
using namespace std;
const long long M = 1e9 + 7;
const long long N = 2000 + 10;
const long long IM = 1e15 + 37;
const long double PI = 3.1415926535897932384;
inline void PP(long long n) { cout << n << " "; }
void PV(vector<long long> v) {
  for (long long i = 0; i < v.size(); i++) cout << v[i] << " ";
  cout << "\n";
}
void PVV(vector<pair<long long, long long> > v) {
  for (long long i = 0; i < v.size(); i++)
    cout << v[i].first << " " << v[i].second << "\n";
}
void PA(long long v[], long long n, long long x = 0) {
  for (long long i = x; i < n + x; i++) cout << v[i] << ' ';
  cout << "\n";
}
void IN(long long a[], long long n, long long x = 0) {
  for (long long i = x; i < n + x; i++) cin >> a[i];
}
inline void op() {}
signed main() {
  op();
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long a, b, k, f;
  cin >> a >> b >> f >> k;
  long long ans = 0, x = 0, y = 0, c = 0;
  long long i = 0;
  long long u = b;
  x = a - f;
  map<long long, long long> m;
  if (f > b) {
    cout << (-1) << "\n";
    return 0;
  }
  u = u - f;
  while (k--) {
    if (k == 0) {
      if (i % 2 == 0) {
        if (u >= x)
          continue;
        else {
          u = b;
          ans++;
          if (u < x) {
            cout << (-1) << "\n";
            return 0;
          }
          continue;
        }
      } else {
        if (u >= f)
          continue;
        else {
          u = b;
          ans++;
          if (u < f) {
            cout << (-1) << "\n";
            return 0;
          }
          continue;
        }
      }
    }
    if (i % 2 == 0) {
      if (u < x + x) {
        u = b;
        ans++;
      }
      if (u < x + x) {
        cout << (-1) << "\n";
        return 0;
      } else {
        u = u - x - x;
        i++;
      }
    } else {
      if (u < f + f) {
        u = b;
        ans++;
      }
      if (u < f + f) {
        cout << (-1) << "\n";
        return 0;
      } else {
        u = u - f - f;
        i++;
      }
    }
  }
  cout << (ans) << "\n";
  return 0;
}
