#include <bits/stdc++.h>
using namespace std;
#pragma comment(linker, "/STACK:20000000")
string problem_name = "a";
void init() { freopen((problem_name + ".in").c_str(), "rt", stdin); }
int a, x, y;
bool ins(double xl, double yl) {
  if (x - 1e-9 <= xl) return 0;
  if (y + 1e-9 >= yl) return 0;
  if (x + 1e-9 >= xl + a) return 0;
  if (y - 1e-9 <= yl - a) return 0;
}
int main() {
  scanf("%d%d%d", &a, &x, &y);
  double h = (a) / 2.;
  if (y <= 0) {
    printf("-1");
    return 0;
  }
  if (y <= 2 * a) {
    if (ins(-h, a))
      printf("1");
    else if (ins(-h, 2 * a))
      printf("2");
    else
      printf("-1");
    return 0;
  }
  y -= a * 2;
  y--;
  int col = y / (2 * a);
  y %= 2 * a;
  int cur = col * 3 + 2;
  if (ins(-a, a - 1))
    printf("%d", cur + 1);
  else if (ins(0, a - 1))
    printf("%d", cur + 2);
  else if (ins(-h, 2 * a - 1))
    printf("%d", cur + 3);
  else
    printf("-1");
  return 0;
}
