/BARA - Bánh rán

              USES CRT;
        var     n,m:int32;
                begin
                    readln(n,m);
                    if (n<=m) then writeln(10)
                    else
                    if (2*n mod m=0) then writeln((2*n div m)*5)
                    else
                    writeln((2*n div m +1)*5);

                end.