#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  long long a, x, y;
  cin >> a >> x >> y;
  if (y % a == 0) {
    cout << -1 << endl;
    return 0;
  }
  long long l = y / a;
  if (l == 0) {
    if (x * 2 >= a || x * 2 <= -a)
      cout << -1 << endl;
    else
      cout << 1 << endl;
    return 0;
  }
  if (l % 2) {
    if (x * 2 >= a || x * 2 <= -a)
      cout << -1 << endl;
    else
      cout << l / 2 * 3 + 2 << endl;
    return 0;
  } else {
    if (x > 0 && x < a) {
      cout << l / 2 * 3 + 1 << endl;
    } else if (x < 0 && x > -a)
      cout << l / 2 * 3 << endl;
    else
      cout << -1;
    return 0;
  }
  return 0;
}
