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