I disagree.
dis?a?gree (ds-gr)
intr.v. dis?a?greed, dis?a?gree?ing, dis?a?grees
1. To fail to correspond: our figures disagree. See Synonyms at differ.
2.
a. To have a differing opinion: She disagrees with him on everything. They say it will rain, but I disagree.
b. To dispute or quarrel.
3. To cause adverse effects: Caffeine disagrees with me.
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http://www.thefreedictionary.com/disagree