How to draw dual y-axis in ggplot with different y ranges?

Question

y1 = c(830.6225, 1051.7180, 1084.5102, 1089.1885, 1184.4557,  969.8625,  881.7043, 1047.6092,  860.3845)
y2 = c(11167.21, 11765.34, 12897.90, 13002.88, 14459.16, 14272.0