1990 * 19911991－1991 * 19901990
= (1991 - 1) * 19911991－1991 * 19901990
= 1991 * (19911991 - 19901990) - 19911991
= 1991 * 10001 - 19911991
= 1991 * 10000 + 1991 - 19910000 - 1991
= 0