If it has no dividends it is the same price as the European option.
The difference between american and european option is that Europeen options will be exercised only at maturity and the US ones at any time...
There is no closed formula for this payoff... the easiest way is to use a binomial tree... have a look in Willmott forum... http://www.wilmott.com/index.cfm?NoC...=Yes&forumid=1
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