issue in upgrading from Ubuntu 12.04 to 14.04

[Mudassir Khan]

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done
mudassir@wazir:~$ ubuntu-bug ubuntu-release-upgrader-core
Using PPAPI flash.
mudassir@wazir:~$ Created new window in existing browser session.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.19
ProcVersionSignature: Ubuntu 3.11.0-26.45~precise1-generic 3.11.10.12
Uname: Linux 3.11.0-26-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.0.1-0ubuntu17.9
Architecture: amd64
Date: Sun Jul 5 15:13:49 2015
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1436084044
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 562
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04.4 LTS "Precise Pangolin" - Release amd64 (20140204)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2015-07-05 (0 days ago)