#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, x, y;
  cin >> a >> x >> y;
  if ((y % a) == 0) {
    cout << -1 << "\n";
    return 0;
  }
  if (y < a) {
    if (x > (-(double)a / 2) && x < ((double)a / 2)) {
      cout << 1 << "\n";
    } else
      cout << -1 << "\n";
    return 0;
  }
  int ypos = y;
  int squareno = y / a;
  squareno++;
  if (squareno & 1) {
    int odds = squareno / 2 + 1;
    odds--;
    int total = squareno + odds;
    if (x == 0 || x <= -(double)a || x >= (double)a) {
      cout << -1 << "\n";
      return 0;
    } else if (x < 0) {
      cout << --total << "\n";
      return 0;
    } else {
      cout << total << "\n";
    }
    return 0;
  } else {
    int odds = (squareno) / 2;
    odds--;
    int total = squareno + odds;
    if (x <= -((double)a / 2) || x >= ((double)a / 2)) {
      cout << -1 << "\n";
    } else {
      cout << total << "\n";
    }
    return 0;
  }
  return 0;
}
