#include <bits/stdc++.h>
using namespace std;
inline bool ascending(int i, int j) { return (i < j); }
inline bool descending(int i, int j) { return (i > j); }
const int MN = 105;
int main() {
  int mod;
  double a;
  long long y, x, div, temp;
  cin >> a >> x >> y;
  mod = y % (int)a;
  if (mod == 0) {
    cout << -1;
    return 0;
  }
  if (y < a) {
    if (x * 2 < a && x * 2 > -(a)) {
      cout << 1;
      return 0;
    } else {
      cout << -1;
      return 0;
    }
  }
  div = (y / a);
  temp = div / 2;
  if (div % 2 == 0) {
    if (x == 0 || x >= a || x <= -a) {
      cout << -1;
      return 0;
    } else if (x > 0 && x <= a) {
      cout << temp * 3 + 1;
      return 0;
    } else {
      cout << temp * 3;
      return 0;
    }
  } else {
    if (x * 2 >= a || x * 2 <= -a) {
      cout << -1;
      return 0;
    } else {
      cout << temp * 3 + 2;
      return 0;
    }
  }
  return 0;
}
