#include <bits/stdc++.h>
using namespace std;
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  a %= b;
  return gcd(b, a);
}
long long int exte_gcd(long long int a, long long int b, long long int& x,
                       long long int& y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  }
  long long int x1, y1;
  long long int d = exte_gcd(b, a % b, x1, y1);
  x = y1;
  y = x1 - y1 * (a / b);
  return d;
}
long long int powe(long long int d, long long int s) {
  if (s == 1)
    return d;
  else if (s == 0)
    return 1;
  else {
    long long int u, v = d;
    for (u = 1; u < s; u++) {
      d = d * v;
    }
    return d;
  }
}
long long int count_divisors(long long int b) {
  long long int i, count = 0, n = 1;
  for (i = 2; i * i <= b; i++) {
    count = 0;
    if (b % i == 0) {
      while (b % i == 0) {
        b = b / i;
        count++;
      }
      n = n * (count + 1);
    }
  }
  if (b > 1) n = 2 * n;
  return n;
}
long long int bigmod(long long int a, long long int b, long long int mod) {
  if (b == 0) return 1;
  long long int x = bigmod(a, b / 2, mod);
  x = (x * x) % mod;
  if (b % 2) x = (x * a) % mod;
  return x;
}
long long int phi(long long int n) {
  double result = n;
  for (long long int p = 2; p * p <= n; ++p) {
    if (n % p == 0) {
      while (n % p == 0) n /= p;
      result *= (1.0 - (1.0 / (double)p));
    }
  }
  if (n > 1) result *= (1.0 - (1.0 / (double)n));
  return (long long int)result;
}
int main() {
  long long int a, b, c, k, ans = 0;
  cin >> a >> b >> c >> k;
  long long int x = b;
  while (k > 0) {
    if (c > x || (a - c) > b) {
      cout << -1;
      return 0;
    }
    if (2 * a - c <= x || (k == 1 && a <= x)) {
      x -= a;
    } else {
      x = b - (a - c);
      ans++;
    }
    c = a - c;
    k--;
  }
  cout << ans << endl;
}
