#include <bits/stdc++.h>
int main() {
  int a, x, y;
  while (scanf(" %d %d %d", &a, &x, &y) == 3) {
    if (x <= -100 || x >= 100 || y % a == 0) {
      printf("-1\n");
      continue;
    }
    double width;
    if (y < a) {
      width = double(a) / 2;
      if (x > -width && x < width)
        printf("%d\n", 1);
      else
        printf("-1\n");
    } else if (y / a % 2 == 0) {
      if (x > -a && x < 0)
        printf("%d\n", 1 + 3 * (y / a / 2) - 1);
      else if (x > 0 && x < a)
        printf("%d\n", 1 + 3 * (y / a / 2));
      else
        printf("-1\n");
    } else {
      width = double(a) / 2;
      if (x > -width && x < width)
        printf("%d\n", 1 + 3 * (y / a / 2) + 1);
      else
        printf("-1\n");
    }
  }
  return 0;
}
