#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, x, y;
  cin >> a >> x >> y;
  int k = 0;
  if (y <= 0) {
    cout << -1;
    return 0;
  }
  int yy = 0, st = 0;
  while (yy < y) {
    k++;
    yy += a;
    if (st % 2 == 0 && st != 0) {
      k++;
    }
    st++;
  }
  st--;
  if (y == yy) {
    cout << -1;
    return 0;
  }
  if (st % 2 == 0 && st != 0) {
    if (abs(x) >= a || x == 0) {
      cout << -1;
      return 0;
    } else {
      if (x < 0) {
        cout << k - 1;
        return 0;
      } else {
        cout << k;
        return 0;
      }
    }
  } else {
    if (abs(x) * 2 >= a) {
      cout << -1;
      return 0;
    } else {
      cout << k;
    }
  }
  return 0;
}
