A=list.files(path ="D:/TELEDETECCION/DATAMOD/A", pattern = "*.tif",full.names=T)
B=list.files(path ="D:/TELEDETECCION/DATAMOD/B", pattern = "*.tif",full.names=T)

slope <- overlay(A, B, fun=function(x, y) {if (is.na(x[1])) { NA } else {lm(x[1] ~ y[1])$coefficients[2]}}