#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, x, y;
  cin >> a >> x >> y;
  int level = (y / a) + (y % a != 0);
  int brick = (level / 2) * 3;
  if (level & 1) {
    if (level != 1) {
      if (x == 0 || abs(x) >= a || y % a == 0)
        return cout << -1, 0;
      else if (x < 0)
        return cout << brick, 0;
      else
        return cout << brick + 1, 0;
    } else {
      if (abs(2 * x) >= a || y % a == 0)
        return cout << -1, 0;
      else
        return cout << 1, 0;
    }
  } else {
    if (abs(2 * x) >= a || y % a == 0)
      return cout << -1, 0;
    else
      return cout << brick - 1, 0;
  }
  return 0;
}
