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