program Dien_tich;
uses crt;
var a,b,h: real;
begin
clrscr;
write('nhap day nho ='); readln(a);
write('nhap day lon ='); readln(b);
write('nhap chieu cao ='); readln(h);
while ( a < 0 ) and ( b < 0) and (c < 0) do
write('nhap lai day nho ='); readln(a);
write('nhap lai day lon ='); readln(b);
write('nhap lai chieu cao ='); readln(h);
writeln;
writeln('Dien tich hinh thang = ', 1/2 * (a+b) * h :4:2);
readln;
end.