calculate Overall Pnsr?

[challanger]

hi,
I'm trying to calculate Avarage and Overall PSNR value for different codecs.To calculate Avarage PSNR
-find all frames PSNR values
-Sum each PNSR
-devide PSNR to total frame number
this value is A-psnr

But I dont know how to calc overall PNSR
Can anybody help me?