Create Roll-up Summary using flows

In this requirement,  we explain how to use flows to update the Number of Contacts for accounts. This feature similar to Roll-up summary concept which we generally have for Master-detail relationship objects.

REQUIREMENT:

In Salesforce, 

• Firstly, a user took an Account and added a contact record to that account. 
• Secondly, the contact saved with user input details. 
• When the users save the contact record then No of Contacts field should be updated to the account record. 
• In addition, this should work when the account has multiple contacts as well.

Account object :

• Field Name: No of Contacts
• Field Type: Number(3,0)