PROGRAMMING:Iteration of radical 2

There is an iterative formula: $$X_ n=\sqrt {x_{ N-1} + 2} $$, no matter how large the initial value (positive number) of X is, after several iterations, it is infinitely close to $$- sqrt2 $$, that is to say, the limit of X sequence is $$- sqrt2 $$. Suppose $$X_ 0 $$= 9999999 (8 9S), program input a positive integer n, output $$X_ The value of N $$(10 decimal places reserved).
###Input example:
```in
eight
```
###Output example:
```out
x[8]=2.0051798692
```
###Input example:
```in
sixteen
```
###Output example:
```out
x[16]=2.0000000790
```







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