#include <stdio.h>

int main(){
    int L, K;
    int divi;

    scanf("%d %d", &L, &K);
    divi = (L/2)/K;
    if(divi == 0){
        printf("0\n");
    }else if((L/2)%K == 0){
        printf("%d\n", K*(divi - 1));
    }else{
        printf("%d\n", K*divi);
    }

    return 0;

}