an=2^n-1
sn=2-1+2^2-1+2^3-1+...+2^n-1
=(2+2^2+2^3+...+2^n)-n
=2^(n+1)-(n+2)＞1020
2^10=1024
s9=2^10-11=1024-11=1013
s10＞1020
n≥10