Sum of GP=a+ar+ar²+...arⁿ⁻¹ where a is the first term, r the common ratio, n is the number of terms.
Call the sum S, then rS=ar+ar²+ar³+...arⁿ.
rS-S=arⁿ-a, so S=(arⁿ-a)/(r-1)=a(rⁿ-1)/(r-1).
If a=2, r=4, then S=2+8+32+128+..., making S=2(4¹³-1)/3 for the first 13 terms.
Therefore S=2(67108863)/3=44739242.