#include <bits/stdc++.h>
using namespace std;
template <class T>
void max_swap(T& a, const T& b) {
  a = max(a, b);
}
template <class T>
void min_swap(T& a, const T& b) {
  a = min(a, b);
}
const double PI = acos(-1.0);
const int dx[] = {0, 1, 0, -1};
const int dy[] = {1, 0, -1, 0};
int main() {
  int a, x, y;
  cin >> a >> x >> y;
  int res = -1;
  int row = y / a - 1;
  if (y % a) {
    if (row == -1 || !(row & 1)) {
      int b = (a + 1) / 2;
      if (abs(x) < b) {
        if (row == -1)
          res = 1;
        else
          res = 2 + 3 * (row / 2);
      }
    } else {
      if (x && abs(x) < a) res = 3 + 3 * (row / 2) + (x > 0);
    }
  }
  cout << res << endl;
}
