def ILLINRows(N): return [list(map(int, input().split())) for i in range(N)]
def twoDimArray(row,column,n): return [[n for j in range(column)] for i in range(row)]
n,d=map(int, input().split())
lis = ILLINRows(n)
dp =twoDimArray(2,n,0)
dp[0][0]=lis[0][0]
dp[1][0]=lis[0][1]-d
for i in range(1,len(dp[0])):
    dp[0][i]=max(dp[0][i-1]+lis[i][0],dp[1][i-1]+lis[i][0]-d)
    dp[1][i]=max(dp[1][i-1]+lis[i][1],dp[0][i-1]+lis[i][1]-d)
print(max(dp[1][n-1],dp[0][n-1]))