Вычислить: p=a(a-n) (a-2n)*…*(a-n2)

Var
a, p:real; 
n, i:integer;
begin 
write('a = '); 
readln(a); 
write('n = '); 
readln(n); 
p := a;
for i := 1 to n do   
p := p * (a - i * n); 
writeln('p = ', p); 
readln;
end.