#include <bits/stdc++.h>
using namespace std;
bool cmp(int a, int b) { return a > b; }
int a;
int x, y;
int main() {
  ios_base::sync_with_stdio(false);
  cin >> a >> x >> y;
  int level = 0, boxNo = 0, start = 0;
  while (start < y) {
    start = start + a;
    level++;
    if (level == 1)
      boxNo = 1;
    else if (level % 2 == 0)
      boxNo += 1;
    else
      boxNo += 2;
  }
  if (y % a == 0)
    cout << "-1" << endl;
  else if (level == 0)
    cout << "-1" << endl;
  else if (level == 1) {
    float v1 = a / (float)2, v2 = -1 * v1;
    if (x < v1 && x > v2)
      cout << 1 << endl;
    else
      cout << "-1" << endl;
  } else {
    if (level % 2 == 0) {
      float v1 = a / (float)2, v2 = -1 * v1;
      if (x < v1 && x > v2)
        cout << boxNo << endl;
      else
        cout << "-1" << endl;
    } else {
      if (x < a && x > 0) {
        cout << boxNo << endl;
      } else if (x > -1 * a && x < 0) {
        cout << boxNo - 1 << endl;
      } else {
        cout << "-1" << endl;
      }
    }
  }
  return 0;
}
