#include<fstream>
#include<algorithm>
using namespace std;
struct gen
{
	int p,c,e;
}; 
gen a[1001];
long g,i,j,w,cost;
int cmp(const gen & a, const gen & b)
{
	return a.e<b.e;
}
int main()
{
	ifstream fin("energii.in");
	ofstream fout("energii.out");
	fin>>g;
	fin>>w;
	for(i=1;i<=g;i++)
	{
		fin>>a[i].p>>a[i].c;
		a[i].e=a[i].p-(a[i].c-a[i].p);
	}
	sort(a+1,a+g+1,cmp);
	for(i=g;i>=1 && w;i--)
	{
		cost+=a[i].c;
		w-=a[i].p;
	}
	if(w>0)
		fout<<-1;
	else
		fout<<cost;
}