Input
4 2
abcd
bcde
bcad
bcde
Output
aaabcde
Input
5 3
bwwwz
hrhdh
sepsp
sqfaf
ajbvw
Output
aaaepfafw
Input
7 6
ypnxnnp
pnxonpm
nxanpou
xnnpmud
nhtdudu
npmuduh
pmutsnz
Output
aaaaaaadudsnz