C++ :

#include<bits/stdc++.h>
using namespace std;
string s1,s2,s3;
int a[1005];
void xx(string s,int ss)
{
	memset(a,0,sizeof(a));
	int si=s.size();
	for(int i=0;i<si/2;i++)
		swap(s[i],s[si-1-i]);
	for(int i=0;i<si;i++)
	{
		a[i]+=(s[i]-'0')*ss;
		a[i+1]+=a[i]/10;
		a[i]=a[i]%10;
	}
	int q=0;
	if(a[si]!=0) si++;
	string s3="";
	swap(s3,s2);
	for(int i=si-1;i>=q;i--)
		s2+=char(a[i]+'0');
}
int main()
{
	cin>>s1;
	if(s1.size()==1)cout<<s1;
	else
	for(;s1.size()!=1;)
	{
		s2+=s1[0];
		for(int i=1;i<s1.size();i++)if(s1[i]!='0')xx(s2,s1[i]-'0');
		swap(s1,s2);
		string fc="";
		swap(fc,s2);
	}
	cout<<s1;
	return 0;
}

Pascal :

var m,n:longint;
i,k,j,s:longint;
begin
  readln(n);
  s:=n;
  k:=n;
  m:=1;
 while k>9 do
   begin
      while s<>0 do
        begin
       j:=s mod 10;
       s:=s div 10;
       m:=m*j;
       k:=m;
       end;
       s:=k;
       m:=1;
     end;
     write(k);
end.

Python :

# coding=utf-8
n=int(input())
while True:
    s=1
    while n>0:
        t=n%10
        s*=t
        n//=10
    if s<10:
        print(s)
        break
    else:
        n=s