#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("energii.in");
ofstream fout("energii.out");
int e[1005], c[1005], a[1005][5005];
int main()
{
    int g,w,i,j,d;
    fin>>g>>w;
    for(i=1;i<=g;i++)
    {
        fin>>e[i]>>c[i];
    }
    for(i=1;i<=g;i++)
    {
        for(j=1;j<=w;j++)
        {
            if(e[i]>=j)
            {
                if(a[i-1][j]==0)
                    a[i][j]=c[i];
                else
                    a[i][j]=min(a[i-1][j],c[i]);
            }
            else
            if(j<=e[i]+e[i-1])
                if(a[i-1][j]==0)
                    a[i][j]=c[i]+c[i-1];
                else
                    a[i][j]=min(a[i-1][j],c[i]+c[i-1]);
        }

    }
    if(a[g][w]==0)
        fout<<-1;
    else
        fout<<a[g][w];
    return 0;
}