为什么错？

var

a,b:array[1..100,0..501] of longint;

m,n,i,j,k,l,x,y:longint;

begin

readln(m,n);

for i:=1 to m do

begin

for j:=1 to n do read(a);

readln;

end;

fillchar(b,sizeof(b),0);

for i:=m-1 downto 1 do

for j:=1 to n do

if (a