#include <bits/stdc++.h>
using namespace std;
int a, x, y;
int test() {
  if (y <= 0) return 0;
  if (y % a == 0) return 0;
  if (x >= a || x <= -a) return 0;
  int left, right;
  if (y < a) {
    if (2 * x > (-1 * a) && 2 * x < a) {
      return 1;
    } else
      return 0;
  }
  y -= a;
  int line = y / a;
  line += 1;
  if (line % 2 == 0) {
    left = -a;
    right = a;
    int mid = 0;
    int n = line / 2;
    if (x == mid) return 0;
    if (x > left && x < mid) return n + 2 * n;
    return n + n * 2 + 1;
  } else {
    if (2 * x <= (-1) * a) return 0;
    if (2 * x >= a) return 0;
    int n = line / 2;
    return n + n * 2 + 2;
  }
}
int main() {
  cin >> a >> x >> y;
  int ans = test();
  if (ans == 0) {
    cout << -1 << endl;
  } else {
    cout << ans << endl;
  }
}
