#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, f, k;
  long long r = 0;
  cin >> a >> b >> f >> k;
  long long c = b;
  long long pos = 0;
  long long ans = 0;
  if (b / a >= k) {
    cout << 0;
  } else if (b < f || b < (a - f)) {
    cout << -1;
  } else {
    for (int i = 0; i < k; i++) {
      if (i != k - 1) {
        if (i & 1) {
          b -= (a - f);
          if (b < 0) {
            pos = -1;
            break;
          }
          if (b < (2 * (f))) {
            ans++;
            b = c;
            b -= (f);
          } else {
            b -= (f);
          }
        } else {
          b -= f;
          if (b < 0) {
            pos = -1;
            break;
          }
          if (b < (2 * (a - f))) {
            ans++;
            b = c;
            b -= (a - f);
          } else {
            b -= (a - f);
          }
        }
      } else {
        if (i & 1) {
          b -= (a - f);
          if (b < 0) {
            pos = -1;
            break;
          } else if (b < f) {
            ans++;
            b = c;
            if (b - f < 0) {
              pos = -1;
              break;
            }
          }
        } else {
          b -= f;
          if (b < 0) {
            pos = -1;
            break;
          }
          if (b < ((a - f))) {
            ans++;
            b = c;
            if (b - (a - f) < 0) {
              pos = -1;
              break;
            }
          }
        }
      }
    }
    if (pos == -1) {
      cout << -1;
    } else {
      cout << ans;
    }
  }
}
