#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
const int inf = (int)1e9;
const double inf1 = 1e-9;
const int ss = (int)1e6 + 3;
int main() {
  int a, x, y;
  cin >> a >> x >> y;
  int z = y / a;
  if (z * a == y) {
    cout << "-1";
    return 0;
  }
  ++z;
  double tmp = (double)a / 2;
  if (y < 2 * a) {
    if (x <= -tmp || x >= tmp) {
      cout << "-1";
      return 0;
    }
    if (z == 1)
      cout << "1";
    else
      cout << "2";
    return 0;
  }
  vector<vector<int> > mas(1000001);
  bool t = true;
  int i = 3;
  int k = 3;
  while (i < 1000001) {
    if (t) {
      mas[i].push_back(k);
      mas[i].push_back(k + 1);
      k += 2;
      t = !t;
    } else {
      mas[i].push_back(k);
      ++k;
      t = !t;
    }
    ++i;
  }
  if (z % 2 == 0) {
    if (x <= -tmp || x >= tmp) {
      cout << "-1";
      return 0;
    }
    cout << mas[z][0];
  } else {
    if (x <= -a || x >= a || x == 0) {
      cout << "-1";
      return 0;
    }
    if (x < 0)
      cout << mas[z][0];
    else
      cout << mas[z][1];
  }
  return 0;
}
