Cap so cong

program cau1;
uses crt;
var n,i,w:integer;
    congsai:real;
    a:array[1..100] of real;

begin
 clrscr;
 write('Nhap n= ');readln(n);
 for i:=1 to n do
  begin
   write('a[',i,']= ');
   readln(a[i]);
  end;
  congsai:=a[2]-a[1];
  w:=0;
 for i:=1 to n do
   if (a[i] <> a[1]+(i-1)*congsai) then inc(w);
 if w<>0 then
  write('Day da nhap khong phai cap so cong')
 else
  write('Day da nhap la cap so cong');
 readln;
end.