import sys
sys.setrecursionlimit(10**6)
inf=float('inf')

N,D=map(int,input().split())

T,K=[0]*N,[0]*N
for i in range(N):
    T[i],K[i]=map(int,input().split())

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

for i in range(N):
    dp[i+1][0]=max(dp[i][0]+T[i],dp[i][1]+T[i]-D)
    dp[i+1][1]=max(dp[i][0]+K[i]-D,dp[i][1]+K[i])
print(max(dp[N][0],dp[N][1]))