#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

int main()
{
	int length = 0;
	int eatLength = 0;

	int eatCnt = 0;

	scanf("%d", &length);
	scanf("%d", &eatLength);

	while (true)
	{
		if (eatCnt + eatLength < length / 2)
		{
			eatCnt += eatLength;
		}
		else
		{
			printf("%d\n", eatCnt);
			return 0;
		}
	}
}