#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, a, b, f, k, sum = 0, c = 0;
  cin >> a >> b >> f >> k;
  n = b;
  if (f <= n)
    n -= f;
  else {
    cout << -1;
    return 0;
  }
  while (c < k) {
    if (c != k - 1) {
      if (2 * (a - f) <= n) {
        c++;
        n -= 2 * (a - f);
      } else {
        if (2 * (a - f) <= b) {
          c++;
          sum++;
          n = b - 2 * (a - f);
        } else {
          cout << -1;
          return 0;
        }
      }
    } else {
      if ((a - f) <= n) {
        c++;
      } else {
        if ((a - f) <= b) {
          sum++;
          c++;
        } else {
          cout << -1;
          return 0;
        }
      }
    }
    if (c == k) {
      cout << sum;
      return 0;
    }
    if (c != k - 1) {
      if (2 * f <= n) {
        c++;
        n -= 2 * f;
      } else {
        if (2 * f <= b) {
          n = b - 2 * f;
          sum++;
          c++;
        } else {
          cout << -1;
          return 0;
        }
      }
    } else {
      if (f <= n) {
        c++;
      } else {
        if (f <= b) {
          c++;
          sum++;
        } else {
          cout << -1;
          return 0;
        }
      }
    }
  }
  cout << sum;
}
