#include <stdio.h>

int main(){
    int n, m, i, flag = 0;
    int old_island, new_island = 0;
    int day_cnt = 0;

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

    while(old_island != 0){
        for(i = m; i > 0; i--){
            old_island -= i;
            if(old_island == 2 || old_island < 0){
                old_island += i;
                flag++;
                continue;
            }
            new_island += i;
            if(new_island == 2){
                old_island += i;
                new_island -= i;
                flag++;
                continue;
            }
            break;
        }
        day_cnt++;
        if(flag == m){
            printf("-1\n");
            break;
        }
        if(old_island == 0 && new_island == n){
            printf("%d\n", day_cnt);
        }
        flag = 0;
    }

    return 0;
}