#include<bits/stdc++.h>
using namespace std;
int a;
double b;
double c[150001];
int r[150001];
double add[150001];
double last[150001];
int main(){
	cin>>a>>b;
	add[0]=b; 
	last[0]=b;
	double k=b;
	for(int i=1;i<=a;i++){
		cin>>c[i]>>r[i];
		last[i]=last[i-1]-c[i]*4;
	}
	for(int i=1;i<=a;i++){
		if(last[r[i]]<c[i]){
			cout<<-1;
			exit(0);
		}
	}
	cout<<fixed<<setprecision(7)<<last[a]*0.2;
	return 0;
}

已AC 但样例 $2$ 没过，求解释