def main():
    N, D = map(int, input().split())
    max_tokyo = 0
    max_kyoto = D * ( -1 )
    for i in range(N):
        T, K = map(int, input().split())
        tmp_max_tokyo = max_tokyo
        tmp_max_kyoto = max_kyoto

        max_tokyo = T + max(tmp_max_tokyo, tmp_max_kyoto - D)
        max_kyoto = K + max(tmp_max_tokyo - D, tmp_max_kyoto)
    #print(max_tokyo, max_kyoto)

    if max_tokyo > max_kyoto:
        print(max_tokyo)
    else:
        print(max_kyoto)
            

if __name__ == '__main__':
    main()