Unable to connect to amazon EC2 instance via PuTTY

I created a new instance of Amazon EC2 in Amazon Web Services (AWS) by using this link:. I even added a SSH rule like this:
Port: 22
Type: SSH
Source: <My IP address>/32
I downloaded the .pem file, converted it into .ppk file by using PuTTYGEN. Then I added host name in PuTTY like this:
ec2-user@<public_DNS>.
I selected default settings, added that .ppk file to PuTTY, logged in and I got this error:
Even trouble shooting link didn't help me.
How can I connect to my amazon instance via PuTTY?