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

N,D = map(int,readline().split())
m = map(int,read().split())
TK = zip(m,m)

x,y = 0,-D
for t,k in TK:
    x,y = max(x,y-D)+t, max(x-D,y)+k

answer = max(x,y)
print(answer)