func numberOfArithmeticSlices(A []int) (cnt int) {
	n := len(A)
	if n < 3 {
		return
	}
	dp := make([]map[int]int, n)
	for i := 0; i < n; i++ {
		dp[i] = make(map[int]int)
	}
	for i := 1; i < n; i++ {
		for j := 0; j < i; j++ {
			diff := A[i] - A[j]
			inc := dp[j][diff] + 1
			cnt += inc - 1
			dp[i][diff] = dp[i][diff] + inc
		}
	}
	return
}