#include <bits/stdc++.h>
using namespace std;
int a, k, coef, cury;
double x, y, dcoef;
int main() {
  cin >> a >> x >> y;
  if (y == 0) {
    cout << -1;
    return 0;
  }
  if (x > -a / 2.0 && x < a / 2.0)
    if (y < a) {
      cout << 1;
      return 0;
    }
  if (y < a)
    if (x <= -a / 2.0 || x >= a / 2.0) {
      cout << -1;
      return 0;
    }
  dcoef = y / (a + 0.0);
  coef = (int)dcoef;
  if (dcoef == (double)coef) {
    cout << "-1";
    return 0;
  }
  if (x > -a / 2.0 && x < a / 2.0)
    if (y < a + a) {
      cout << 2;
      return 0;
    }
  if (coef % 2 == 0 && (x <= -a || x >= a)) {
    cout << "-1";
    return 0;
  }
  if (coef % 2 == 0 && x == 0) {
    cout << "-1";
    return 0;
  }
  if (coef % 2 == 1 && (x <= -a / 2.0 || x >= a / 2.0)) {
    cout << "-1";
    return 0;
  }
  if (coef % 2 == 0) {
    cury = a + a;
    k = 3;
    while (1) {
      if (cury + a > y) {
        if (x < 0)
          cout << k;
        else
          cout << k + 1;
        return 0;
      }
      cury += (a + a);
      k += 3;
    }
  }
  cury = a + a + a;
  k = 5;
  while (1) {
    if (cury + a > y) {
      cout << k << endl;
      return 0;
    }
    cury += (a + a);
    k += 3;
  }
  return 0;
}
