#include <bits/stdc++.h>
using namespace std;
int a, x, y, left_number, ri8_number, mid_number;
void number_calculate_odd(int row) {
  left_number = (row / 2) * 3;
  ri8_number = (row / 2) * 3 + 1;
}
void number_calculate_even(int row) {
  int d = row - 1;
  mid_number = row / 2 + d;
}
int main() {
  scanf("%d %d %d", &a, &x, &y);
  if (y % a == 0) {
    printf("-1\n");
  } else {
    int row = (ceil)((double)y / a);
    if (row % 2 == 0) {
      number_calculate_even(row);
      if (x > -((float)a / 2) && x < ((float)a / 2))
        printf("%d\n", mid_number);
      else
        printf("-1\n");
    } else {
      if (row == 1) {
        mid_number = 1;
        if (x > -((float)a / 2) && x < ((float)a / 2))
          printf("1\n");
        else
          printf("-1\n");
      } else {
        number_calculate_odd(row);
        if (x > -a && x < 0) {
          printf("%d\n", left_number);
        } else if (x > 0 && x < a) {
          printf("%d\n", ri8_number);
        } else {
          printf("-1\n");
        }
      }
    }
  }
}
