#include <bits/stdc++.h>

using namespace std;

int a, b, f, k, now, c;

int main(){
  cin >> a >> b >> f >> k;
  now = b;
  for(int i = 0; i < k; i++){
    if(i & 1){
      now -= (a-f);
      if(now < 0){
        cout << -1;
        return 0;
      }
      else{
        if(i == k - 1){
          if(a - f > now){
            now = b;
            c++;
          }
        }
        else if((f) * 2 > now){
          if((f) * 2 > b){
            cout << -1;
            return 0;
          }
          else{
            now = b;
            c++;
          }
        }
      }
        now -= (f);
      }
    else{
      now -= f;
      if(now < 0){
        cout << -1;
        return 0;
      }
      else{
        if(i == k - 1){
          if(a - f > now){
            now = b;
            c++;
          }
        }
        else if((a - f) * 2 > now){
          if((a - f) * 2 > b){
            cout << -1;
            return 0;
          }
          else{
            now = b;
            c++;
          }
        }
        now -= (a - f);
      }
    }
  }
  cout << c;
}