#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, f, k;
  scanf("%d%d%d%d", &a, &b, &f, &k);
  if (f > b) {
    printf("-1\n");
    return 0;
  }
  int jc = 0;
  int fuel0 = b;
  int fuel_first = b - f;
  int fuel_left = 0;
  int fuel_right = 0;
  int fuel_last;
  int fuel = b;
  int isLeft = 1;
  int ans = 0;
  if (k == 1) {
    if (a <= b) {
      printf("0\n");
      return 0;
    } else {
      if (b < (a - f)) {
        printf("-1\n");
        return 0;
      } else {
        printf("1\n");
        return 0;
      }
    }
  }
  if (k == 2) {
    if ((b - f) < 2 * (a - f)) {
      if (b < 2 * (a - f)) {
        printf("-1\n");
        return 0;
      } else {
        if ((b - 2 * (a - f)) < f) {
          printf("2\n");
          return 0;
        } else {
          printf("1\n");
          return 0;
        }
      }
    } else {
      if ((b - f - 2 * (a - f)) < f) {
        printf("1\n");
        return 0;
      } else {
        printf("0\n");
        return 0;
      }
    }
  }
  if (k > 2) {
    fuel = b - f;
    for (int i = 1; i < k; i++) {
      if (i % 2 == 1) {
        if (fuel < 2 * (a - f)) {
          if (b < 2 * (a - f)) {
            printf("-1\n");
            return 0;
          } else {
            ans++;
            fuel = b - 2 * (a - f);
          }
        } else {
          fuel = fuel - 2 * (a - f);
        }
      } else {
        if (fuel < 2 * f) {
          if (b < 2 * f) {
            printf("-1\n");
            return 0;
          } else {
            ans++;
            fuel = b - 2 * f;
          }
        } else {
          fuel = fuel - 2 * f;
        }
      }
    }
  }
  if (k % 2 == 0) {
    if (fuel < f) {
      if (b < f) {
        printf("-1\n");
        return 0;
      } else {
        ans++;
        printf("%d\n", ans);
        return 0;
      }
    } else {
      printf("%d\n", ans);
      return 0;
    }
  } else {
    if (fuel >= a - f) {
      printf("%d\n", ans);
      return 0;
    } else {
      if (b < a - f) {
        printf("-1\n");
        return 0;
      } else {
        ans++;
        printf("%d\n", ans);
        return 0;
      }
    }
  }
}
