n,d=map(int,input().split())
ns=[list(map(int,input().split())) for i in range(n)]
s=[[0]*2 for i in range(n+1)]

for i in range(n):
    for j in range(2):
        if i == 0 and j == 1:
            continue
        for k in range(2):
            if j == k:
                s[i+1][k] = max(s[i+1][k],s[i][j]+ns[i][k])
            else:
                s[i+1][k] = max(s[i+1][k],s[i][j]+ns[i][k]-d)
                
                
print(max(s[-1]))