#include <bits/stdc++.h>
using namespace std;
int work() {
  int a, x, y;
  cin >> a >> x >> y;
  if (y % a == 0) return -1;
  int h = y / a;
  if (h == 0) {
    if (2 * x > -a && 2 * x < a)
      return 1;
    else
      return -1;
  }
  if (h % 2 != 0) {
    if (2 * x > -a && 2 * x < a) {
      return (y - a) / (2 * a) * 3 + 2;
    }
  } else {
    if (x > -a && x < a) {
      int n = ((y - a) / (2 * a) + 1) * 3 + 1;
      if (x < 0)
        return n - 1;
      else if (x > 0)
        return n;
    }
  }
  return -1;
}
int main() { cout << work() << endl; }
