#1-C(365,n)n!/(365)^n
init=365
k=0
for(n in 4:init){
  pk=1-choose(365,n)*factorial(n)/365^n
  if(pk>=0.5){
    k=n; 
    break;
  }
}
print(k)

choose(n,k)
factorial(5)
choose(4,2) #组合
#sqrt(x)