#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const long long INFF = 0x3f3f3f3f3f3f3f3f;
const double pi = acos(-1.0);
const double eps = 1e-9;
const long long mod = 1e9 + 7;
long long read() {
  long long x = 0, f = 1;
  char ch = getchar();
  while (ch<'0' | ch> '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
void Out(int aa) {
  if (aa > 9) Out(aa / 10);
  putchar(aa % 10 + '0');
}
long long a[10010];
queue<int> q;
int main() {
  long long A = read(), b = read(), f = read(), k = read();
  a[1] = f;
  for (int i = 2; i <= k; i++) {
    if (i % 2 == 0)
      a[i] = a[i - 1] + (A - f) * 2;
    else
      a[i] = a[i - 1] + f * 2;
  }
  if (k % 2) {
    k++;
    a[k] = a[k - 1] + A - f;
  } else {
    k++;
    a[k] = a[k - 1] + f;
  }
  long long pos = 0, tank = b;
  long long ans = 0;
  for (int i = 1; i <= k; i++) {
    long long d = a[i] - pos;
    while (tank < d) {
      if (q.empty()) {
        printf("-1\n");
        return 0;
      }
      tank = b;
      q.pop();
      ans++;
    }
    tank -= d;
    q.push(1);
    pos = a[i];
  }
  cout << ans;
  return 0;
}
