求原字串与其反字串的最长公共子序列，最后结果用n减去LCS的长度即可
program poj1159;
var
s1,s2:ansistring;
n,i,j:integer;
f:array[0..1,0..5001] of integer;
function max(p,q:integer):integer;
begin
if p>q then max:=p
else max:=q;
end;
begin
readln(n);
readln(s1);
fillchar(f,sizeof(f),0);
s2:='';
for i:=n downto 1 do
s2:=s2+s1[i];
for i:=1 to n do
begin
for j:=1 to n do
if s1[i]=s2[j] then f[1,j]:=f[0,j-1]+1
else f[1,j]:=max(f[0,j],f[1,j-1]);
f[0]:=f[1];
end;
writeln(n-f[1,n]);
end.
