import std.stdio;
import std.algorithm;
import std.conv;
import std.range;

void main()
{
    auto input = readln.splitter.map!(to!int).array;
    int yuu = 0;
    for (auto l = input[0], k = input[1] * 2; l > k; l -= k) {
        ++yuu;
    }
    writeln(yuu * input[1]);
}