#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, x, y;
  scanf("%d%d%d", &a, &x, &y);
  int t = y / a;
  int r = y % a;
  if (r == 0) {
    printf("-1\n");
    return 0;
  }
  if (t == 0) {
    if (x < a / 2.0 && x > -1 * (a / 2.0))
      printf("1\n");
    else
      printf("-1\n");
  } else if ((t - 1) % 2 == 0) {
    if (x < a / 2.0 && x > -1 * (a / 2.0))
      printf("%d\n", 2 + 3 * (t - 1) / 2);
    else
      printf("-1\n");
  } else {
    if (x < a && x > 0)
      printf("%d\n", 4 + (t - 1) / 2 * 3);
    else if (x < 0 && x > -1 * a)
      printf("%d\n", 3 + (t - 1) / 2 * 3);
    else
      printf("-1\n");
  }
}
