#include <bits/stdc++.h>
using namespace std;
int a, x, y;
void ReadInput() { cin >> a >> x >> y; }
int main() {
  ReadInput();
  if (y <= 0 || x >= a || x <= -a || y % a == 0) {
    cout << -1 << endl;
    return 0;
  }
  int num = y / a;
  int ted = num;
  ted /= 5;
  int T = ceil((double)a / 2);
  if (num < 5) {
    switch (num) {
      case 0:
        if (x > -T && x < T)
          cout << ted * 7 + 1 << endl;
        else
          cout << -1 << endl;
        return 0;
      case 1:
        if (x > -T && x < T)
          cout << ted * 7 + 2 << endl;
        else
          cout << -1 << endl;
        return 0;
      case 2:
        if (x > -1 * a && x < 0)
          cout << ted * 7 + 3 << endl;
        else if (x > 0 && x < a)
          cout << ted * 7 + 4 << endl;
        else
          cout << -1 << endl;
        return 0;
      case 3:
        if (x > -T && x < T)
          cout << ted * 7 + 5 << endl;
        else
          cout << -1 << endl;
        return 0;
      case 4:
        if (x > -1 * a && x < 0)
          cout << ted * 7 + 6 << endl;
        else if (x > 0 && x < a)
          cout << ted * 7 + 7 << endl;
        else
          cout << -1 << endl;
        return 0;
    }
  }
  num -= 5;
  ted = num;
  ted /= 2;
  num %= 2;
  switch (num) {
    case 0:
      if (x > -T && x < T)
        cout << ted * 3 + 1 + 7 << endl;
      else
        cout << -1 << endl;
      return 0;
    case 1:
      if (x > -1 * a && x < 0)
        cout << ted * 3 + 2 + 7 << endl;
      else if (x > 0 && x < a)
        cout << ted * 3 + 3 + 7 << endl;
      else
        cout << -1 << endl;
      return 0;
  }
  return 0;
}
