#include <stdio.h>
int main(void){
    int n;  //  number of animals
    int m;  //  number of ships

    scanf("%d %d",&n,&m);

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

    return 0;
}