#include <bits/stdc++.h>
using namespace std;
int dx[8] = {0, 0, 1, -1, 1, 1, -1, -1};
int dy[8] = {1, -1, 0, 0, 1, -1, 1, -1};
long long int k;
int main() {
  long long int a, b, f, k, fl, ans;
  while (~scanf("%lld", &a)) {
    scanf("%lld %lld %lld", &b, &f, &k);
    fl = ans = 0;
    fl = !fl;
    long long int dis = a, oil = b;
    for (long long int i = 1; i <= k; i++) {
      long long int ex = (i % 2 != 0) ? a - f : f;
      long long int ex2 = (i % 2 == 0) ? a - f : f;
      if (i == k) {
        if (oil < ex2)
          fl = 0;
        else if (oil < dis && b >= (dis - ex2))
          ans++;
        else if (oil < dis)
          fl = 0;
      } else {
        if (oil < ex2) {
          fl = 0;
        } else if (oil >= dis + ex) {
          oil -= dis;
        } else {
          oil -= ex2;
          oil = b;
          oil -= (i % 2 == 0) ? (f) : (a - f);
          ans++;
          long long int curdis = (i % 2 == 0) ? a - f : f;
        }
      }
    }
    (fl == 1) ? printf("%lld\n", ans) : printf("-1\n");
  }
  return 0;
}
