#include <bits/stdc++.h>
using namespace std;
int a, x, y, cell = -1;
void solve() {
  int nr = 1;
  if (!(y % a)) return;
  int r = y / a;
  if (!r) {
    cell = 1;
  } else if (r % 2) {
    cell = 2 + (r / 2) * 3;
  } else {
    nr = 2;
    cell = 1 + (r / 2) * 3;
  }
  if (nr == 1) {
    if (!(x > ((float)a / -2) && x < ((float)a / 2))) cell = -1;
  } else {
    if (x == 0) {
      cell = -1;
    } else if (x > (-1 * a) && x < 0) {
      --cell;
    } else if (x > 0 && x < a) {
    } else {
      cell = -1;
    }
  }
}
int main() {
  cin >> a >> x >> y;
  if (y > 0) solve();
  cout << cell;
  return 0;
}
