#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  int a, x, y;
  cin >> a >> x >> y;
  if (y % a == 0) {
    cout << "-1";
    return 0;
  }
  int h = a / 2;
  if (a % 2 == 0) h--;
  if (y <= 2 * a) {
    if (-h <= x && x <= h) {
      if (y > a)
        cout << "2";
      else
        cout << "1";
    } else {
      cout << "-1";
    }
    return 0;
  }
  int l = (y - 2 * a) / a;
  if (l % 2 == 0) {
    if (x == 0) {
      cout << "-1";
      return 0;
    }
    if (!(-a < x && x < a)) {
      cout << "-1";
      return 0;
    }
    int below = 2 + l + (l + 1) / 2;
    if (x < 0)
      cout << below + 1 << "\n";
    else
      cout << below + 2 << "\n";
  } else {
    if (!(-h <= x && x <= h)) {
      cout << "-1";
      return 0;
    }
    int below = 2 + l + (l + 1) / 2;
    cout << below + 1 << "\n";
  }
  return 0;
}
