Поменять местами первый и последний символы строки

uses
  crt;
const
  n = 10;
var
  i,min,max: integer;
  a: array [1..n] of integer;
begin
  clrscr;
  for i:= 1 to n do
  begin
    a[i]:=random(30);
    write(a[i],' ');
  end;
  
  min:=a[1];
  max:=a[1];
  for i:= 2 to n do
  begin
    if a[i]>max then
      max:=a[i];
    if a[i]<min then
      min:=a[i];
  end;
  write('Ðàçíîñòü = ',max-min);
  readkey;
end.

uses
  crt;
var
  i: integer;
  st: string;
  c: char;
begin
  clrscr;
  write('Ââåäèòå ñòðîêó: ');
  readln(st);
  c:=st[1];
  st[1]:=st[length(st)];
  st[length(st)]:=c;
  write('Ðåçóëüòàò: ',st);
end.

uses crt;
var
a:array[1..3,1..3] of integer;
s,i,j:integer;
begin
clrscr;
 
for i:=1 to 3 do
for j:=1 to 4 do
begin
     writeln('a^2+1[',i,',',j,']');
     readln(a^2+1[i,j]);
end;
 
for i:=1 to 3 do
begin
writeln;
for j:=1 to 4 do write(a^2+1[i,j],'  ');
end;
 
s:=0;
for i:=1 to 3 do
for j:=1 to 3 do
     if i=j then s:=s*a^2+1[i,j];
writeln;
writeln('suma=',s);
readln;
end.

uses crt;
var
a:array[1..3,1..3] of integer;
s,i,j:integer;
begin
clrscr;
 
for i:=1 to 3 do
for j:=1 to 4 do
begin
writeln('a[',i,',',j,']');
readln(a[i,j]);
end;
 
for i:=1 to 3 do
begin
writeln;
for j:=1 to 4 do write(a[i,j],' ');
end;
 
s:=0;
for i:=1 to 3 do
for j:=1 to 3 do
if i=j then s:=s*a[i,j];
writeln;
writeln('suma=',s);
readln;
end.

uses crt;
var
a:array[1..3,1..3] of integer;
s,i,j:integer;
begin
clrscr;
 
for i:=1 to 3 do
for j:=1 to 4 do
begin
writeln('a[',i,',',j,']');
readln(a[i,j]);
end;
 
for i:=1 to 3 do
begin
writeln;
for j:=1 to 4 do write(a[i,j],' ');
end;
 
s:=0;
for i:=1 to 3 do
for j:=1 to 3 do
if i=j then s:=s*a[i,j];
writeln;
writeln('suma=',s);
readln;
end.

uses crt;
const n=10;
var a:array[1..n,1..n] of integer;
    i,j,p:integer;
begin
ClrScr;
Randomize;
for i:=1 to n do
 begin
  for j:=1 to n do
   begin
    a[i,j]:=random(20)-10;
    Write(a[i,j]:4);
   end;
  Writeln;
 end;
Writeln;
p:=1;
for i:=1 to n do
 for j:=1 to n do
  if i=j then
   if a[i,j] mod 2<>0 then
    p:=p*a[i,j];
Writeln('P=',p);
Readln;
end.

program tmp;
uses crt;
const n=5;
var
   masx:array[1..n,1..n] of integer;
   mas:array[1..n] of integer;
   i,j,k,p,m:integer;
 
begin
clrscr;
randomize;
for i:=1 to n do
    for j:=1 to n do masx[i,j]:=random(10)-5;
for i:=1 to n do begin
    for j:=1 to n do write(masx[i,j]:2);
                     writeln;
                     end;
writeln;
k:=0;
for i:=1 to n do
    for j:=1 to n do
        if i=j then begin
                    k:=k+1;
                    mas[k]:=masx[i,j];
                    end;
p:=1;
for m:=1 to k do
if m mod 2<>0 then p:=p*mas[m];
writeln('proizvedenie: ',p);
readkey;
end.