import sys

sys.setrecursionlimit(10**6)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.buffer.readline())
def MI(): return map(int, sys.stdin.buffer.readline().split())
def MI1(): return map(int1, sys.stdin.buffer.readline().split())
def LI(): return list(map(int, sys.stdin.buffer.readline().split()))
def LI1(): return list(map(int1, sys.stdin.buffer.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def BI(): return sys.stdin.buffer.readline().rstrip()
def SI(): return sys.stdin.buffer.readline().rstrip().decode()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [[(-1, 0), (1, 0)], [(0, -1), (0, 1)]]
inf = 10**16
# md = 998244353
md = 10**9+7

n,d=MI()
tk=LLI(n)
dp=[[-inf]*2 for _ in range(n+1)]
dp[0][0]=0

def chmax(i,j,val):
    if val>dp[i][j]:dp[i][j]=val

for i,income in enumerate(tk):
    for j in range(2):
        pre=dp[i][j]
        if pre==-inf:continue
        for nj in range(2):
            profit=income[nj]
            if j!=nj:profit-=d
            chmax(i+1,nj,pre+profit)

print(max(dp[-1]))