program ConvexPolygonChecker;

Uses
  SysUtils;

Var
  N: Integer;
  IsIncorrect: Boolean;
  XValues: Array Of Integer;
  YValues: Array Of Integer;
  I: Integer;
  HasPositive, HasNegative: Boolean;
  CrossProduct: Double;

Begin
  Write('This program determines whether a polygon is convex based on the coordinates of its points entered by the user.');
  Repeat
    IsIncorrect := False;
    Write('Enter the number of sides of the polygon: ');
    Try
      ReadLn(N);
      If (N < 3) Then
      Begin
        WriteLn('Polygon must have at least 3 sides.');
        IsIncorrect := True;
      End
      Else If (N > 1000) Then
      Begin
        WriteLn('Number of sides is too big. Please, choose a value less than 1000.');
        IsIncorrect := True;
      End;
    Except
        WriteLn('Invalid input.');
        IsIncorrect := True;
    End;
  Until (Not IsIncorrect);

  SetLength(XValues, N);
  SetLength(YValues, N);

  For I := 0 To N - 1 Do
  Begin
    Repeat
      IsIncorrect := False;

      Write('Enter x value for point ', I + 1, ': ');
      Try
        ReadLn(XValues[I]);
        if (XValues[I] < -1000) or (XValues[I] > 1000) Then
        Begin
          WriteLn('X value is out of range [-1000, 1000]. Please, choose a different value.');
          IsIncorrect := True;
        End;
      Except
        Begin
          WriteLn('Invalid input!');
          IsIncorrect := True;
        End;
      End;

      Write('Enter y value for point ', I + 1, ': ');
      Try
        ReadLn(YValues[I]);
        If (YValues[I] < -1000) or (YValues[I] > 1000) then
        Begin
          WriteLn('Y value is out of range [-1000, 1000]. Please, choose a different value.');
          IsIncorrect := True;
        End;
      Except
          WriteLn('Invalid input!');
          IsIncorrect := True;
      End;
    Until (Not IsIncorrect);
  End;

  HasPositive := False;
  HasNegative := False;

  For I := 0 To N - 1 Do
  Begin
    CrossProduct := (XValues[(I + 1) mod N] - XValues[I]) * (YValues[(I + 2) mod N] - YValues[(I + 1) mod N]) -
                    (YValues[(I + 1) mod N] - YValues[I]) * (XValues[(I + 2) mod N] - XValues[(I + 1) mod N]);

    If CrossProduct > 0 Then
      HasPositive := True
    Else If CrossProduct < 0 Then
      HasNegative := True;
  End;

  If (HasPositive And HasNegative) Or (Not HasPositive And Not HasNegative) Then
    Begin
        WriteLn('The polygon is not convex.');
        Readln;
    End
  Else
    WriteLn('The polygon is convex.');
    Readln;
End.