read N
i=0
read n
fv=`echo $n | cut -c1`
f=$fv
while [ $i -lt $(($N-1)) ]
do
    read n
    if [ $f -eq $fv ]; then
        f=`echo $n | grep $fv | cut -c1`

    fi
    i=$(($i+1))
done
if [ $f -eq $fv ]; then
    echo "YES"
else
    echo "NO"
fi