How can I solve this using bigmod algorithm?

If the series is 1+a+a^2+a^3+a^4+a^5 then this will be equal to (1+a^2+a^4)+a(1+a^2+a^4). How can I apply bigmod here!

BigMod: this is for calculating the mod of a large number like 2^100 or 10^18.

ll bigmod(ll a, ll b, ll m) { if(b==0) return 1; if(b%2==1) { ll p1 = a%m; ll p2 = bigmod(a,b-1,m); return (p1*p2)%m; } if(b%2==0) { ll h = bigmod(a,b/2,m); return (h*h)%m; } }