Площадь многоугольника, используя координаты

Program Ploshad_Mnogougolnika;
Uses crt;
Var p,s,min: real;
    n: integer;
    x,y: array[1..21] of real;
 
Procedure Vvod;
Var i: integer;
Begin
 write('Vvedite kolichestvo yglov mnogougolnika n=');
 readln(n);
 for i:= 1 to n do
  begin
   writeln('vvedite kkordinatu N ',i);
   write('x[',i,']=');
   readln(x[i]);
   write('y[',i,']=');
   readln(y[i]);
  end;
  x[n+1]:=x[1];
  y[n+1]:=y[1];
End;
 
Procedure Perenos;
Var i: integer;
Begin
 min:=y[1];
 for i:= 2 to n do
  if min >y[i] then
   min:=y[i];
  for i:=1 to n+1 do
   y[i]:=y[i]-min;
End;
 
Procedure Ploshad;
Var i: integer;
Begin
 s:=0;
 for i:= 1 to n do
  s:= s+((y[i+1]+y[i])*(x[i+1]-x[i])/2);
end;
 
Procedure Vyvod;
Begin
 writeln('Ploshad mnogougolnika S=',abs(s):1:2);
 readln;
end;
 
Begin
 clrscr;
 Vvod;
 Perenos;
 Ploshad;
 Vyvod;
end.

Program dd;
Uses crt;
Var s1,ss1,x1,y1,p2,pp,a,b,c: real;
    n,i,code: integer;
    x,y: array[1..1000] of real;
    ss,sss:string;
    squarein,squareout:text;
 
Begin
Assign(squarein,'square.in');
Assign(squareout,'square.out');
 
rewrite(squareout);
 clrscr;
 
reset(squarein);
readln(squarein,ss);
val(ss,n,code);
delete(ss,1,pos(' ',ss));
 
for i:= 1 to n do
begin
readln (squarein,sss);
val(copy(sss,1,pos(' ',sss)-1),x1,code);
delete(sss,1,pos(' ',sss));
x[i]:=x1;
val(copy(sss,1,length(sss)),y1,code);
y[i]:=y1;
s1:=0;
end;
 for i:= 2 to n-1 do
 begin
 a:=sqrt(sqr(x[1]-x[i])+sqr(y[1]-y[i]));
 b:=sqrt(sqr(x[1]-x[i+1])+sqr(y[1]-y[i+1]));
 c:=sqrt(sqr(x[i]-x[i+1])+sqr(y[i]-y[i+1]));
 p2:=a+b+c;
 pp:=p2/2;
 ss1:=sqrt(pp*(pp-a)*(pp-b)*(pp-c));
 S1:=s1+ss1;
 end;
 writeln(squareout,s1:1:5);
 
 close(squarein);
 close(squareout);
end.