N,D = map(int,input().split())
TK = [list(map(int,input().split())) for i in range(N)]

dp = [ [ 0 for j in range(2)] for i in  range(1+N)]
dp[0][1] = -D

for i in range(N):
    for j in range(2):
        if j == 0:
            dp[i+1][0] = max(dp[i][0]+TK[i][0],dp[i][1]+TK[i][1] - D)
        if j == 1:
            dp[i+1][1] = max(dp[i][1]+TK[i][1],dp[i][0]+TK[i][0] - D)

print(max(dp[N][0] , dp[N][1]))