import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

n, m = na()
cd = [na() for i in range(n)]
cd = sorted(cd, key = lambda x:-x[0])

c, d = zip(*cd)

N = 23
inf = 10 ** 18
# inf = float("inf")
dp = [[-inf for j in range((m>>i) + 1)] for i in range(N)]
dp[0][0] = 0

A = 0
for i in range(n):
    if c[i] == 0:
        A += d[i]
        continue
    for j in range(N-2, -1, -1):
        C = c[i]
        D = d[i]
        k = 0
        Z = (m >> j & 1 == 0)
        for k in range(len(dp[j+1]) * 2 - C - Z-1, -1, -1):
            if dp[j][k] == -inf:
                continue
            z = (k + C + Z) // 2
            dp[j+1][z] = max(dp[j+1][z], dp[j][k] + D)
    # for i in dp:
    #     print(*i)
    # print()
ans = -inf
for i in range(N):
    ans = max(ans, max(dp[i]))
# print(z)
print(ans + A)