int findInvalidInterimBlockIndex(Keccak256Hash sessionId) throws Exception{
        List<Sha256Hash> hashesFromContract = ethWrapper.getBlockHashesBySession(sessionId);
        Keccak256Hash superblockId = ethWrapper.getSuperblockIdBySession(sessionId);
        BigInteger height = ethWrapper.getSuperblockHeight(superblockId);
        // find local superblock based on height of superblock being challenged
        Superblock superblock = superblockChain.getSuperblockByHeight(height.longValue());
        if(superblock == null)
            throw new Exception("Superblock {} not found in local chain at height {} " + superblockId + height.longValue());



        List<Sha256Hash> localHashes = superblock.getSyscoinBlockHashes();
        if(localHashes.size() != superblockChain.SUPERBLOCK_DURATION)
            throw new Exception("Local superblock must have 60 hashes, we found: " + localHashes.size());
        if(hashesFromContract.size() != superblockChain.SUPERBLOCK_DURATION)
            throw new Exception("Stored superblock must have 60 hashes, we found: " + hashesFromContract.size());

        // we want to ensure block->prev of first header matches previous superblock's last hash to check continuation of superblocks
        StoredBlock firstBlock = syscoinWrapper.getBlock(hashesFromContract.get(0));
        // if we don't have the block representing the first hash of the superblock then it must be a bad block to us, so we should ask submitter to prove 0th block
        if(firstBlock == null) {
            return 0;
        }
        // check first block prev hash matches prev superblock last block hash
        Sha256Hash lastBlockHash = ethWrapper.getSuperblockLastHash(superblock.getParentId());
        // add last block of prev superblock to the hashesFromContract as we will step through the local hashes and compare prev blocks to the contract hashes
        hashesFromContract.add(0, lastBlockHash);
        for (int i = 0; i < localHashes.size(); i++) {
            StoredBlock block = syscoinWrapper.getBlock(localHashes.get(i));
            if(block == null) {
                throw new Exception("Cannot find local block at index: " + i);
            }
            if(block.getHeader().getPrevBlockHash() != hashesFromContract.get(i)){
                return i;
            }
        }
        // if all matches then just return -1 meaning we don't have to check interim block for this challenge
        return -1;
    }