#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  long long a, x, y;
  cin >> a >> x >> y;
  if (y % a == 0) {
    cout << -1;
    return 0;
  }
  long long tmp, tmp2 = y / a;
  if (y < 2 * a)
    tmp = 1;
  else
    tmp = (long long)(tmp2 % 2 == 0) + 1;
  if (-a * tmp < 2 * x && a * tmp > 2 * x) {
    long long res = tmp2 + (tmp2 - 1) / 2;
    if (tmp == 1) {
      cout << res + 1 << endl;
    } else {
      if (x == 0) {
        cout << -1;
        return 0;
      }
      if (x > 0) {
        cout << res + 2;
      } else
        cout << res + 1;
    }
  } else
    cout << -1;
}
