#include <bits/stdc++.h>
using namespace std;
int main() {
  double a, x, y;
  x = 1, y = 1.1;
  cin >> a >> x >> y;
  x = x / a;
  bool possible = false;
  y = y / a;
  if (y > 0 and y < 1) {
    if (x > -0.5 and x < 0.5) {
      cout << 1 << endl;
      possible = true;
    }
  }
  int j = 1;
  y = y - 1;
  for (int i = 0; i <= y; i++) {
    if (i & 1) {
      if (y > i and y < i + 1) {
        if (x > 0 and x < 1) {
          cout << j + 2 << endl;
          possible = true;
          break;
        } else if (x > -1 and x < 0) {
          cout << j + 1 << endl;
          possible = true;
          break;
        }
      }
      j += 2;
    } else {
      if (y > i and y < i + 1) {
        if (x > -0.5 and x < 0.5) {
          cout << j + 1 << endl;
          possible = true;
          break;
        }
      }
      j++;
    }
  }
  if (!possible) {
    cout << -1 << endl;
  }
}
