#include <bits/stdc++.h>   using namespace std;   int main() {     int a, b, f, k;     cin >> a >> b >> f >> k;     int now = b;     int ret = 0;     for (int i = 1; i <= k; i++) {       if (i % 2 == 1) {         if (now < f) {           return puts( -1 ), 0;         }         if (i == k) {           if (now < a) {             ret++;           }           if (b < a - f) {             return puts( -1 ), 0;           }         } else {           if (now < a + a - f) {             ret++;             now = b - (a - f);           } else {             now = now - a;           }         }       } else {         if (now < a - f) {           return puts( -1 ), 0;         }         if (i == k) {           if (now < a) {             ret++;           }           if (b < f) {             return puts( -1 ), 0;           }         } else {           if (now < a + f) {             ret++;             now = b - f;           } else {             now = now - a;           }         }       }     }     cout << ret << endl;     return 0;   }   