#include <bits/stdc++.h>
using namespace std;
int a, x, y;
int checkX1() {
  if (-a < 2 * x && 2 * x < a) return 0;
  return -1;
}
int checkX2() {
  if (-a < x && x < 0) return 0;
  if (0 < x && x < a) return 1;
  return -1;
}
int solve() {
  if (y <= 0) return -1;
  if (y % a == 0) return -1;
  y /= a;
  int t = y / 2 * 3;
  if (y == 0) {
    if (checkX1() == -1) return -1;
    return 1;
  }
  if (y % 2 == 0) {
    if (checkX2() == -1) return -1;
    return t + checkX2();
  } else {
    if (checkX1() == -1) return -1;
    return t + 2;
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin >> a >> x >> y;
  cout << solve() << endl;
  return 0;
}
