#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, x, y, rem_y, res;
  cin >> a >> x >> y;
  if (abs(x) < (a / 2.0) && y > 0 && y < a)
    cout << "1\n";
  else if (abs(x) < a && y > a) {
    rem_y = y % a;
    if (rem_y == 0)
      cout << "-1\n";
    else {
      y /= a;
      y++;
      if (y % 2 == 0) {
        if (abs(x) < (a / 2.0)) {
          res = (y - 2) / 2;
          y += res;
          cout << y << endl;
        } else
          cout << "-1\n";
      } else {
        res = (y - 3) / 2;
        if (x < 0) {
          y += res;
          cout << y << endl;
        } else if (x > 0) {
          y += (res + 1);
          cout << y << endl;
        } else
          cout << "-1\n";
      }
    }
  } else
    cout << "-1\n";
}
