Calculating p-value for 1-tailed test in a linear model

Hello,

I'm having trouble figuring out how to calculate a p-value for a 1-tailed
test of beta_1 in a linear model fit using command lm.  My model has only 1
continuous, predictor variable.  I want to test the null hypothesis beta_1
is>= 0.  I can calculate the p-value for a 2-tailed test using the code
"2*pt(-abs(t-value), df=degrees.freedom)", where t-value and degrees.freedom
are values provided in the summary of the lm.  The resulting p-value is the
same as provided by the summary of the lm for beta_1.  I'm unsure how to
change my calculation of the p-value for a 1-tailed test.

Thanks for your assistance,

Andy