#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, x, y, max = 0, h;
  float rx, lx;
  cin >> a >> x >> y;
  if (y == 0) {
    cout << -1;
    return 0;
  }
  if (y % a == 0) {
    cout << -1;
  } else {
    max = ceil((float)y / (float)a);
    h = max;
    max = ((max - 1) / 2) * 2 + ((max - 1) - ((max - 1) / 2)) + 1;
    if ((int)(h - 1) % 2 == 0 && h > 1) {
      if ((x > 0 && x < a))
        cout << max;
      else if (x < 0 && x > (-1 * a))
        cout << max - 1;
      else
        cout << -1;
    } else {
      rx = abs(((float)a) / 2);
      if ((x > (-1 * rx) && x < rx))
        cout << max;
      else
        cout << -1;
    }
  }
}
