C++ :

#include<bits/stdc++.h>
using namespace std;
int a[10000],b[10000],c[10000];
string add(string s1,string s2)
{
	int len1=s1.size();
	int len2=s2.size();
	int len3=max(len1,len2);
	for(int i=len1-1,j=0;i>=0;i--,j++)
	   a[j]=s1[i]-'0';
	for(int i=len2-1,j=0;i>=0;i--,j++)
	    b[j]=s2[i]-'0';
	int jw=0;
	for(int i=0;i<len3;i++)
	{
		c[i]=a[i]+b[i]+jw;
		jw=c[i]/10;
		c[i]=c[i]%10;
	}
	if(jw)c[len3++]=1;
	string s3;
	for(int i=len3-1;i>=0;i--)
	s3+=c[i]+'0';
	return s3;
}
int main(){
   string s1,s2;
   cin>>s1>>s2;
   cout<<add(s1,s2);
    return 0;
}

Pascal :

var s1,s2:string;
    x1,x2,x:array[0..1000]of longint;
    i,j,l,l1,l2:longint;
begin
 readln(s1);
 readln(s2);
 l1:=length(s1);
 l2:=length(s2);
 for i:=1 to l1 do x1[i]:=ord(s1[l1+1-i])-48;
 for i:=1 to l2 do x2[i]:=ord(s2[l2+1-i])-48;
 if l1>l2 then l:=l1 else l:=l2;
 j:=0;
 for i:=1 to l do
  begin
   x[i]:=(x1[i]+x2[i]+j)mod 10;
   j:=(x1[i]+x2[i]+j) div 10;
  end;
 if j>0 then begin inc(l);x[l]:=j;end;
 for i:=l downto 1 do write(x[i]);
end.