哪里错了？拿了40分

program luogu_1002(input,output);
var 
  a1:array[1..8]of longint=(2,1,-1,-2,-2,-1,1,2);
  a2:array[1..8]of longint=(1,2,2,1,-1,-2,-2,-1);
  f:array[-1..21,-1..21]of longint;
  a:array[-1..21,-1..21]of boolean;
  n,m,i,j,x,y,p,q:longint;
begin
  for i:=-1 to 21 do
    for j:=-1 to 21 do
      a[i,j]:=true;
  readln(n,m,x,y);
  a[x,y]:=false;
  for i:=1 to 8 do
    begin
      p:=x+a1[i]; q:=y+a2[i];
      if (p>=0)and(p<=n)and(q>=0)and(q<=m) then a[p,q]:=false;
    end;
  for i:=1 to n do f[i,0]:=1;
  for j:=1 to m do f[0,j]:=1;
  for i:=1 to n do
    for j:=1 to m do
      begin
        if a[i,j]=false then f[i,j]:=0
          else f[i,j]:=f[(i-1),j]+f[i,(j-1)];
      end;
writeln(f[n,m]);
end.