"Alchemy: the science of understanding, deconstructing, and reconstructing matter. However, it is not an all-powerful art; it is impossible to create something out of nothing. If one wishes to obtain something, something of equal value must be given. This is the Law of Equivalent Exchange, the basis of all alchemy. In accordance with this law, there is a taboo among alchemists: human transmutation is strictly forbidden - for what could equal the value of a human soul...?" (Narrator, Fullmetal Alchemist Brotherhood)
The answer is a right arm! It says so in the FIRST VOLUME!