I have a dataframe like this 

```
sample <- data.frame(x1=c(1,2),y1=c(2,1), x2=c(2,4),y2=c(3,4),x3=c(5,2),y3=c(1,6))

```

How can I operate pairwise (sum up x1&y1, x2&y2, x3&y3, 2 columns at a time) to create 3 new columns sum1, sum2, sum3? Thanks