PROGRAMMING:Step tariff
In order to promote the residents to save electricity, a provincial power company implements the "ladder price". The electricity price of the residents who install one meter for one household is divided into two "ladder": if the monthly electricity consumption is less than 50 kWh (including 50 KWH), the electricity price is 0.53 yuan / kWh; If the electricity consumption exceeds 50 kwh, the electricity price will be increased by 0.05 yuan / kWh. Please write a program to calculate the electricity charge.
###Input format:
Enter the monthly electricity consumption (unit: KWH) of a user in one line.
###Output format:
Output the electricity charge (yuan) that should be paid by the user in one line, and keep two decimal places in the result, such as "cost = value of electricity charge payable"; If the power consumption is less than 0, "invalid value!" is output.
###Input sample 1:
```in
ten
```
###Output sample 1:
```out
cost = 5.30
```
###Input sample 2:
```
one hundred
```
###Output sample 2:
```
cost = 55.50
```
answer：If there is no answer, please comment
###Input format:
Enter the monthly electricity consumption (unit: KWH) of a user in one line.
###Output format:
Output the electricity charge (yuan) that should be paid by the user in one line, and keep two decimal places in the result, such as "cost = value of electricity charge payable"; If the power consumption is less than 0, "invalid value!" is output.
###Input sample 1:
```in
ten
```
###Output sample 1:
```out
cost = 5.30
```
###Input sample 2:
```
one hundred
```
###Output sample 2:
```
cost = 55.50
```
answer：If there is no answer, please comment