program problem;
var a,b,c,d:integer;
begin
read(a);read(b);read(c);
d:=(a+5*b)*(c-b*b)/4
writeln(d);
end.
program problem;
var a,b,c,d:integer;
begin
read(a);read(b);read(c);
d:=(a+5*b)*(c-b*b)/4
writeln(d);
end.