n=scan("stdin",,,,",")
m=(x=1:n[2])%o%(y=1:n)
i=96
l=length
while(n<-l(diag(x%o%y))){m[x[z<-1:n],y[z]]=i=i+1
l(x)>n&&(x=x[-z])||(y=y[-z])}
write(apply(m,1,intToUtf8),1)