It is the _________ in Britain that young people often do not shake hands when they meet someone for the first time.

A．custom

B．habit

C．hobby

D．favour

有以下程序
#include ＜stdio．h＞
main()
int m，n；
scanf("%d%d"，&m，&n)；
while(m! =n)
while(m＞n)m=m-n；
while(m＜n)n=n-m；

printf("%d\n"，m)；

程序运行后，当输入14 63＜回车＞时，输出结果是______。