from collections import defaultdict
import copy
n,m=map(int,input().split())
CD=[list(map(int,input().split())) for _ in range(n)]
CD.sort(reverse=True)
D=defaultdict(list)
D[0].append((0,0))
for c,d in CD:
    D1=copy.deepcopy(D)
    for i,J in D.items():
        for a,b in J:
            if i+c*pow(2,b)<=m:
                D1[i+c*pow(2,b)].append((a+d,b+1))
    D=D1
ans=0
for j in D.values():
    for a,b in j:
        ans=max(ans,a)
print(ans)