#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, x, y;
  cin >> a >> x >> y;
  if (y % a == 0)
    cout << "-1\n";
  else {
    int lvl = y / a;
    if (lvl == 0) {
      if (x * 2 > -a && x * 2 < a)
        cout << "1\n";
      else
        cout << "-1\n";
    } else {
      lvl--;
      int st = 2 + (lvl / 2) * 3;
      if (lvl % 2 == 0) {
        if (x * 2 > -a && x * 2 < a)
          cout << st << "\n";
        else
          cout << "-1\n";
      } else {
        if (x > -a && x < 0)
          cout << st + 1 << "\n";
        else if (x > 0 && x < a)
          cout << st + 2 << "\n";
        else
          cout << "-1\n";
      }
    }
  }
  return 0;
}
