cannot upgrade to 15.04

im on 14.10

ubuntu suggest me to update to 15.04
even tough its not an LTS I agree to install.

it got stuck (tried about a dozen time) at :

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: ubuntu-release-upgrader-core 1:14.10.10
ProcVersionSignature: Ubuntu 3.16.0-37.51-generic 3.16.7-ckt9
Uname: Linux 3.16.0-37-generic x86_64
ApportVersion: 2.14.7-0ubuntu8.4
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Wed May 20 10:34:36 2015
InstallationDate: Installed on 2014-06-04 (349 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to utopic on 2015-05-20 (0 days ago)