#include <bits/stdc++.h>
using namespace std;
int main() {
  int k, t;
  double a, x, y;
  while (cin >> a >> x >> y) {
    if ((int)y % (int)a == 0)
      cout << -1 << endl;
    else {
      if (y < 2 * a) {
        if (x >= a / 2 || x <= -a / 2)
          cout << -1 << endl;
        else if (y < a)
          cout << 1 << endl;
        else
          cout << 2 << endl;
      } else {
        if (x >= a || x <= -a)
          cout << -1 << endl;
        else {
          y = y - 2 * a;
          k = (int)y / a + 1;
          if (k % 2 == 1) {
            t = 3 + (k - 1) / 2 * 3;
            if (x == 0)
              cout << -1 << endl;
            else {
              if (x > 0)
                cout << t + 1 << endl;
              else
                cout << t << endl;
            }
          } else {
            t = 5 + (k - 2) / 2 * 3;
            if (x >= a / 2 || x <= -a / 2)
              cout << -1 << endl;
            else {
              cout << t << endl;
            }
          }
        }
      }
    }
  }
  return 0;
}
