Skip file

What i have is a for loop to name files. I want it to skip over the file if it doesn't exist. Is there a way to do that?

s=seq(from = chron("03/15/2012"), to = chron("06/15/2012"))
day=format(as.Date(s), "%Y%m%d")
for (k in 1:length(day)){
B1=read.csv(paste("S:/file_", day[k], ".csv", sep=""))
Date=strptime(B1[,1], format="%m/%d/%Y %I:%M:%S %p")
print(B1) }