Pairs that make a price

Write pairs_summing(prices, target) that returns how many pairs of positions (i, j) with i < j have prices[i] + prices[j] == target.

pairs_summing([10, 30, 20, 30], 40)  ->  2     (10+30 at positions 0,1 and 0,3)
pairs_summing([5], 10)               ->  0

def pairs_summing(prices, target):
    ...