Maxcompute factorial-Pascal

First, provide the Code:

program fac;var   i,j,n,len: integer;   rad,c,r: longint;   buff: array[1..16000] of integer;   num: string[5];begin   writeln('calculate n!');   write('n=?');   read(n);   rad:=10000;   len:=1;   buff[1]:=1;   for i:=1 to n do   begin      c:=0;      for j:= 1 to len do      begin        r := buff[j] * i +c;        buff[j] := r mod rad;        c := r div rad;      end;      if c>0 then      begin        len:=len+1;        buff[len]:=c;      end;   end;   write('n!=',buff[len]);   for i:=len-1 downto 1 do   begin       str(buff[i],num);       if length(num)<4 then          write(copy('0000',1,4-length(num)));       write(buff[i]);   end;END.

Note:
Development Environment: Free Pascal 2.4 (a DOS program, similar to Turbo Pascal)
Download Page: http://down.tech.sina.com.cn/page/46658.html
Free Pascal settings:
1. the DOS window code page is set to 437 (USA). Otherwise, garbled characters will occur.
2. Options-> compiler mode is set to Turbo Pascal compatible
Learning materials:
Pascal language training courses, download page http://www.05sun.com/downinfo/5433.html

Download, install free Pascal. It takes about two hours to learn, compile, and debug the program.
Problems:
1. Data overflow caused by the definition of variable R as integer
2. If the for keyword is deduplicated and changed to downto, everything is normal.