uses crt;
var dl, db, h, s : real;
begin
clrscr;
write('nhap day lon va day be : ');readln(dl, db);
write('nhap chieu cao hinh thang : ');readln(h);
if (dl > 0) and (db > 0) and (dl > db) then
begin
s := ((dl + db) / 2) * h;
writeln('dien tich : ', s:2:1);
end;
readln
end.