#include <bits/stdc++.h>
using ll = long long;
int main() {
  ll a, x, y;
  std::cin >> a >> x >> y;
  ll h = y / a;
  ll ans = 0;
  if (y <= 0 || y % a == 0) {
    std::cout << -1 << std::endl;
    return 0;
  }
  if (h == 0) {
    if (-a < 2 * x && 2 * x < a) {
      ans = 1;
    } else {
      ans = -1;
    }
  } else {
    if (h % 2 == 0) {
      if (-a < x && x < a) {
        ll base = 1 + (1 + 2) * (h / 2);
        if (x < 0)
          ans = base - 1;
        else if (x == 0)
          ans = -1;
        else
          ans = base;
      } else {
        ans = -1;
      }
    } else {
      if (-a < 2 * x && 2 * x < a) {
        ans = 1 + (1 + 2) * (h / 2) + 1;
      } else {
        ans = -1;
      }
    }
  }
  std::cout << ans << std::endl;
  return 0;
}
