from sys import stdin
n, d, *indata = map(int, stdin.read().split())
offset = 0
p = []
q = []
for i in range(n):
    s, t = indata[offset + 2*i],indata[offset + 2*i+1]
    p.append(s)
    q.append(t)

minp = min(p)
ans = -10**18
for i in range(n):
    for j in range(n):
        if i != j:
            karimin = -p[i]
            val = -p[i]
            val += q[i] - p[j]
            karimin = min(karimin,val)
            val += q[j]
            if d >= 3:
                val = val * ((d-3) // 2)
                val += -p[i]
                karimin = min(karimin,val)
                val += q[i] - p[j]
                karimin = min(karimin,val)
                val += q[j] - minp
                karimin = min(karimin,val)
            ans = max(ans,karimin)
print("{}".format(ans))