C++ :

#include<iostream>
#include<algorithm>
using namespace std;
int main()
{ 
	string s,t;
	string Mars="0123456789abcdefghij";
	int i;
	while(cin>>s>>t)
	{
		int a[100]={0},b[100]={0},c[101]={0};
		int m=s.length();
		reverse(s.begin(),s.end());
		int n=s.length();
		reverse(t.begin(),t.end());
		for(i=0;i<m;i++)
		{
			if(s[i]>='a')
				a[i]=s[i]-'a'+10;
			else
				a[i]=s[i]-'0';
		}
		for(i=0;i<n;i++)
		{
			if(t[i]>='a')
				b[i]=t[i]-'a'+10;
			else
				b[i]=t[i]-'0';
		}
		int len=max(m,n);
		for(i=0;i<len;i++)
		{
			c[i+1]=(a[i]+b[i]+c[i])/20;
			c[i]=(a[i]+b[i]+c[i])%20;
		}
		if(c[i]==0) i--;
		for(;i>=0;i--)
		{
			cout<<Mars[c[i]];
		}
		cout<<endl;
	}
	return 0;
}