模板 - 欧拉函数
int euler (int n){ //返回euler(n)
int res = n,a = n;
for (int i = 2;i*i <= a;i ++){
if (a%i == 0){
res = res/i*(i-1);//先进行除法是为了防止中间数据的溢出
while (a%i == 0) a /= i;
}
}
if (a > 1) res = res/a*(a-1);
return res;
}