#include<bits/stdc++.h>
#define L(i, j, k) for(int i = (j); i <= (k); ++i) 
#define R(i, j, k) for(int i = (j); i >= (k); --i)
#define vi vector < int > 
#define ll long long 
#define me(a, x) memset(a, x, sizeof(a)) 
#define sz(a) ((int) (a).size())
using namespace std;
const int N = 1e4 + 7;
int n;
int s[N], l[N], r[N];
double a, v, t;
int main() {
	ios :: sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	cin >> n >> a;
	L(i, 0, n) 
		cin >> s[i];
	L(i, 0, n) 
		cin >> l[i] >> r[i];
	L(i, 1, n) 
		l[i] = max(l[i], l[i - 1]);
	R(i, n - 1, 0) 
		r[i] = min(r[i], r[i + 1]);
	v = t = 0; 
	L(i, 1, n) {
		double S = s[i] - s[i - 1];
		double nt = (sqrt(v * v + 2 * a * S) - v) / a;
		if(t + nt < l[i]) {
			double kt = l[i] - t;
			double rs = kt * kt * a / 2;
			if(rs < S) {
				v = (S - rs) / kt + a * kt;
			} else {
				v = sqrt(S * 2 * a);
			}
			t = l[i];
		} else {
			v += nt * a, t += nt; 
		}
		if(t > r[i] + 1e-8) {
			cout << -1 << '\n';
			continue;
		}
	}
	cout.precision(12); cout << fixed;
	cout << t << '\n';
	return 0;
}
/*
0.5857864376

*/