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