import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))


n,m = list(map(int, input().split()))
ab = [list(map(int, input().split())) for _ in range(n)]
v0 = [max(a,b) for a,b in ab]
v1 = [max(m*a, (m-1)*a+b, b) for a,b in ab]
cummax = 0
cum0 = 0
cum1 = 0
ans = -float("inf")
for j in range(n):
    cum0 += v0[j]
    ans = max(ans, cum0 + cummax)
#     print(cum0, cummax)
    cum1 += v1[j]
    cummax = max(cummax, -cum0 + cum1)
ans = max(ans, cum0 + cummax)
print(ans)