#include <bits/stdc++.h>
using namespace std;
int main(int argc, char** argv) {
  int a, x, y;
  cin >> a;
  cin >> x;
  cin >> y;
  if (y > 0 && y < a) {
    if (x < (double)a / 2 && x > (double)-a / 2)
      cout << 1 << endl;
    else
      cout << -1 << endl;
    return 0;
  }
  int r = (y - a) % a;
  int q = (y - a) / (2 * a);
  int mody = (y - a) % (2 * a);
  if (y < 0 || r <= 0) {
    cout << -1 << endl;
    return 0;
  }
  if (mody < a) {
    if (x < (double)a / 2 && x > (double)-a / 2)
      cout << 1 + 3 * q + 1 << endl;
    else
      cout << -1 << endl;
  } else {
    if (x > (double)-1 * a && x < 0)
      cout << 1 + q * 3 + 2 << endl;
    else if (x > 0 && x < a)
      cout << 1 + q * 3 + 3 << endl;
    else
      cout << -1 << endl;
  }
  return 0;
}
