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