N,D=map(int,input().split())
T=[]
K=[]
for i in range(N):
    t,k=map(int,input().split())
    T.append(t)
    K.append(k)

dp=[[0]*(N+1) for i in range(N+1)]
dp[1][0]=T[0]
dp[1][1]=K[0]-D
for i in range(1,N):
    dp[i+1][0]=dp[i][0]+T[i]
    for j in range(N):
        if j<=i:
            if j%2:
                dp[i+1][j+1]=max(dp[i][j+1]+T[i],dp[i][j]+T[i]-D)
            else:
                dp[i+1][j+1]=max(dp[i][j+1]+K[i],dp[i][j]+K[i]-D)

print(max(dp[N]))