#include <bits/stdc++.h>
using namespace std;
int mark1[26];
int mark2[26];
int main() {
  float a, x, y;
  int a2, y2;
  cin >> a >> x >> y;
  a2 = a;
  y2 = y;
  int level = y + a - 1;
  level = level / a;
  int ans;
  if (level % 2 == 1) {
    ans = level / 2 * 3 + 1;
  } else
    ans = (level - 1) / 2 * 3 + 2;
  if (level == 0 || y2 % a2 == 0)
    cout << -1;
  else {
    if (level == 1) {
      x = abs(x);
      if (x < float(a / 2))
        cout << ans;
      else
        cout << -1;
    } else {
      if (level % 2 == 0) {
        x = abs(x);
        if (x < float(a / 2))
          cout << ans;
        else
          cout << -1;
      } else {
        if (x != 0 && abs(x) < a) {
          if (x < 0)
            cout << ans - 1;
          else
            cout << ans;
        } else
          cout << -1;
      }
    }
  }
}
