var a,d:real;
n:integer;
begin
writeln ('Vvedite pervii chiclo');
readln (a);
writeln ('Vvedite raznost');
readln (d);
writeln ('Vvedite kol-vo');
readln (n);
write ('Summa arif. progressii ravna ',((2*a+(n-1)*d)/2)*n);
end.