N,D=map(int,input().split())
tn, kn = 0, -D
for _ in range(N):
    T, K = map(int,input().split())
    tt, kk, tk, kt = tn+T, kn+K, kn-D+T, tn-D+K
    tn = tt if tt>tk else tk
    kn = kk if kk>kt else kt
print(tn if tn>kn else kn)