#include <bits/stdc++.h>
int main() {
  long long int a, x, y, ans = 0, k, s;
  scanf("%I64d%I64d%I64d", &a, &x, &y);
  if ((x >= a) || (x <= -a) || (y == 0) || (y % a == 0))
    printf("-1");
  else {
    long long int n = (y / a) + 1;
    long long int pos2 = n * a;
    if ((y == pos2))
      ans = 0;
    else if (y < pos2) {
      if ((n % 2 == 1) && (n != 1)) {
        k = n / 2;
        if (x > 0) ans = k + n;
        if (x < 0) ans = k - 1 + n;
        if (x == 0) ans = -1;
      } else if ((n % 2 == 0) && (x > -a / 2.0) && (x < a / 2.0)) {
        s = n / 2;
        ans = 2 + (s - 1) * 3;
      } else if ((n == 1) && (x > -a / 2.0) && (x < a / 2.0))
        ans = 1;
      if (ans == 0) ans = -1;
      printf("%I64d", ans);
    }
  }
}
