Greg pad the pad screen nm rectangle cell black white well consider pad row number integer 1 n similarly pad column number integer 1 m leave rightgreg think pad screen display cave follow condition hold there segment lr 1lrn row ll1r exactly black cell row white cell there row number t ltr pair row number j lijt set column black cell row column black cell subset set column black cell row j column black cell similarly pair row number j tijr set column black cell row j column black cell subset set column black cell row column black cell Greg wonder way paint cave pad two way consider distinct cell distinct color pictureshelp Greg