import std.stdio, std.conv, std.string, std.range, std.array, std.algorithm;

int main(){
	auto I = readln.strip.split.to!(real[]);
	I[0]/=2;
	((I[0]/I[1] - (I[0]%I[1]==0 ? 1 : 0) ).to!(int)*I[1]).to!int.writeln;
	return 0;
}