program Dien_tich_hinh_thang;

var

s,a,b,h:real;

write ('a='):readln(a);

write('b='):readln(b);

write('h='):readln(h);

s:=(a+b)*h*1/2;

write ('dien tich hinh thang la',s:8:3);

readln;

end.