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