import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N, M = map(int, readline().split())
m = map(int, read().split())
AB = zip(m, m)

dp = [0, 0, 0]  # 0回、1回、M回
for a, b in AB:
    newdp = [0, 0, 0]
    newdp[0] = max(dp)
    newdp[1] = max(dp[1:]) + max(a, b)
    full = a * (M - 1) + max(a, b)
    newdp[2] = dp[2] + max(full, b)
    dp = newdp
print(max(dp))