n = read_line.to_i
a = read_line.split.map &.to_i
m = read_line.to_i.pred
ave = a.sum / n
val1 = a.map { |i| (i - ave).abs ** 2 }
val2 = Math.sqrt(val1.sum / n)
val3 = (ave - a[m]).abs*10/val2
puts (ave < a[m] ? 50 + val3 : 50 - val3).to_i