N,D=map(int,input().split())
B="T"
ans=0
for i in range(N):
    T,K=map(int,input().split())
    if B=="T":
        ans=max(ans+T,ans+K-D)
        if T<K-D:
            B="K"
    else:
        ans=max(ans+T-D,ans+K)
        if K<T-D:
            B="T"
print(ans)