how to get value for previous month in looker studio?

[brndp]

I have a field in my data that represents number of employees in the end of the month (called ending headcount) and also its month year. i want to make a calculated fields that will count number of employees in the beginning of the month which has the same value as ending headcount in the previous month . For example: number of employees in the beginning of the month February = ending headcount on january 

how to write the formula for count number of employees in the beginning of the month (same as ending headcount for previous month) in calculated field on looker studio?

Attached the data looks like this.