```
function gcd(a, b:Longword):Longword;
var
t:Longword;
begin
while b <> 0 do
begin
t := b;
b := a mod b;
a := t;
end;
gcd := a;
end;
```

This function can be used for calculating Euler's Totient function:
```
function fi(a:Longword):Longword;
var
i:Longword;
count:Longword;
begin
if a = 1 then
begin
count := 1;
Exit
end;
count := 0;
for i := 1 to a do
begin
if gcd(i, a) = 1 then
count := count +1;
end;
fi := count;
end;
```

**N.B.**Keep in mind that this algorithms are far from optimal.