#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;
  }
  y /= a;
  if (y == 0) {
    if (a & 1 && x >= -a / 2 && x <= a / 2)
      cout << 1;
    else if (a % 2 == 0 && x > -a / 2 && x < a / 2)
      cout << 1;
    else
      cout << "-1";
  } else if ((y - 1) & 1) {
    if (x == 0 || x <= -a || x >= a)
      cout << "-1";
    else if (x < 0)
      cout << ((y - 2) / 2) * 3 + 3;
    else
      cout << ((y - 2) / 2) * 3 + 4;
  } else {
    if (a & 1 && x >= -a / 2 && x <= a / 2)
      cout << 2 + ((y - 1) / 2) * 3;
    else if (a % 2 == 0 && x > -a / 2 && x < a / 2)
      cout << 2 + ((y - 1) / 2) * 3;
    else
      cout << "-1";
  }
  return 0;
}
