#include<fstream.h>

int a[1002],b[1002],c[1002];

int main()
{
   long int s,s1,s2,min=-1;
   int t,g,w,ok=1,i;
   ifstream fin("energii.in");
   ofstream fout("energii.out");
   fin>>g>>w;
   s=0;
   for(i=1;i<=g;i++)
   {   fin>>a[i]>>b[i]; s+=a[i];}
   fin.close();
   if(s<w) fout<<-1;
   else
   {
      while(ok)
      {  s1=s2=0;
         c[g]++;
         ok=0;
         for(i=g;i>=1;i--)
         {
            t=c[i];
            if(c[i]>1) 
             {  c[i]=t%2;
            c[i-1]+=t/2;}
            if(!c[i]) ok=1;
            else {s1+=a[i]; s2+=b[i];}
         }  
         if(s1>=w)
            if(min<0) min=s2; 
            else if(min>=s2) min=s2;
      }
   }
   fout<<min;
   fout.close();
   return 0;
}