[R] An issue regarding to gradient

[luke1022]

The following code will get me a curve plot:
cutoff <- seq(1,7,0.25)
Sensitivity <- 1 - pnorm(cutoff, 5, 0.8)
Specificity <- pnorm(cutoff, 3, 1.2)
plot(1-Specificity,Sensitivity,main = "ROC curve",type = "o")

How do I get a gradient of a particular point on that curve?
Any packages/functions allow me to do that?  

Thank you

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