#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int a, x, y, ans;
  cin >> a >> x >> y;
  if (y < a && y != 0) {
    if (x < (double)a / 2 && x > -(double)a / 2) {
      ans = 1;
    } else
      ans = -1;
  } else {
    y = y - a;
    if (y % a != 0) {
      if ((y / a) % 2 == 0 && x < (double)a / 2 && x > -(double)a / 2) {
        ans = (double)(y / a) * (1.5) + 2;
      } else if (x < (double)a && x > 0 && (y / a) % 2 != 0)
        ans = (double)((y / a) - 1) * (1.5) + 4;
      else if (x > -(double)a && x < 0 && (y / a) % 2 != 0)
        ans = (double)((y / a) - 1) * (1.5) + 3;
      else
        ans = -1;
    } else
      ans = -1;
  }
  cout << ans << endl;
  return 0;
}
