#include <bits/stdc++.h>
using namespace std;
void fl() {}
int main() {
  fl();
  int a, x, y, yy;
  cin >> a >> x >> y;
  if (y % a == 0) return puts("-1"), 0;
  yy = y / a + 1;
  if (yy == 1 or !(yy % 2)) {
    if (x * 2 <= -a or x * 2 >= a) return puts("-1"), 0;
    cout << (yy == 1 ? 1 : 3 * yy / 2 - 1);
  } else {
    if (x > -a and x < 0)
      cout << yy / 2 * 3;
    else if (x > 0 and x < a)
      cout << yy / 2 * 3 + 1;
    else
      puts("-1");
  }
  return 0;
}
