import sys
sys.setrecursionlimit(10000)

# データを読み込む
n, d = map(int, input().split())

t = []
k = []

for i in range(n):
    ti, ki = map(int, input().split())
    t.append(ti)
    k.append(ki)

labor_condition = [t, k]


# positionにいる場合の、m日目以降の稼ぎを最大化する
def search(position, m):
    if m ==n-1:
        return max(labor_condition[position][m], 
                   labor_condition[abs(position - 1)][m] - d)
    
    else:
        return max(labor_condition[position][m] + search(position, m+1), 
                   labor_condition[abs(position - 1)][m] - d + search(abs(position - 1), m+1))

print(search(0, 0))