#include <stdio.h>

int main() {
  int n, m;
  scanf("%d %d", &n, &m);

  if (n<=m) {
    printf("1\n");
  } else if (n%2==0 && n/2<=m) {
    printf("%d\n", n/(n/2));
  } else if (n%m==0 && n/2<=m) {
    printf("%d\n", n/m);
  } else {
    printf("-1\n");
  }
  return 0;
}