#include<stdio.h>

void run(void){
  int n,m;
  scanf("%d%d",&n,&m);
  if(n<=m){
    printf("1\n");
  } else if(2*m>=n){
    if(n%2==1){
      printf("-1\n");
    } else {
      printf("2\n");
    }
  } else {
    printf("-1\n");
  }
  return;
}

int main(void){
  run();
  return 0;
}