Generate n-th Fibonacci string that is defined with the next recurrent formula:
f(0)="a";
f(1)="b";
f(n)=f(n−1)+f(n−2), where "+" operation means concatenation
For example, f(3)=f(2)+f(1)=(f(1)+f(0))+f(1)="b"+"a"+"b"="bab".
One integer n (0≤n≤20).
Print the n-th Fibonacci string.