#!/usr/bin/env python3

w, M, l, h = (x := list(map(int, input().split())))[1], [tuple(map(int, input().split())) for _ in range(x[0])], 2, 1e18
while h - l > 1e-6 or print(m):
    l, h = (l, m) if sum(max(0, p * ((m := (l + h) / 2) - 2 * t)) for p, t in M) >= w else (m, h)