> d=data.frame(y1=c(1,3,2,7),y2=c(2,4,5,2))
> ybar = mean(c(d$y1, d$y2))
> ybar
[1] 3.25

> t(d$y1-ybar) %*% (d$y1-ybar)
      [,1]
[1,] 20.75
> t(d$y2-ybar) %*% (d$y2-ybar)
     [,1]
[1,] 6.75

> unlist(d)
y11 y12 y13 y14 y21 y22 y23 y24
  1   3   2   7   2   4   5   2
> sum( (unlist(d) - mean(unlist(d)) )^2 )
[1] 27.5