#include <stdio.h>

int main(){
    int n, m;;

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

    return 0;
}