#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int a, x, y;
  cin >> a >> x >> y;
  pair<int, int> cor = {-a / 2, a / 2}, bigcor = {-a, a};
  if ((x < bigcor.second && x > bigcor.first) && (y > 0)) {
    if (y % a == 0) {
      cout << "-1\n";
    } else {
      int pos = y / a;
      if (pos == 0) {
        if (abs(x * 2) >= a) {
          cout << "-1\n";
        } else {
          cout << "1\n";
        }
      } else if (pos & 1) {
        if (abs(x * 2) >= a) {
          cout << "-1\n";
        } else {
          cout << (pos / 2) * 3 + 2 << endl;
        }
      } else {
        if (x > 0) {
          cout << 1 + (pos / 2) * 3 << endl;
        } else if (x == 0) {
          cout << "-1\n";
        } else {
          cout << (pos / 2) * 3 << endl;
        }
      }
    }
  } else {
    cout << "-1\n";
  }
}
