Efficiently sum across multiple columns in R

I have the following condensed data set:

a<-as.data.frame(c(2000:2005))
a$Col1<-c(1:6)
a$Col2<-seq(2,12,2)

colnames(a)<-c("year","Col1","Col2")

for (i in 1:2){
  a[[paste("Var_", i, sep="")]]<-i*a[[paste("Col", i, sep="")]]
}

I would like to sum the columns Var1 and Var2, which I use:

a$sum<-a$Var_1 + a$Var_2

In reality my data set is much larger – I would like to sum from Var_1 to Var_n (n can be upto 20). There must be a more efficient way to do this than:

 a$sum<-a$Var_1 + ... + a$Var_n