#include <stdio.h>
int main(void){
    int l, k;
    int times = 0;
    scanf( "%d %d", &l, &k );
    times = l % ( k * 2 ) == 0 ? ( l / ( k * 2 ) ) - 1 : ( l / ( k * 2 ) );
    printf( "%d", k * times );
}