Big number summation(UVA 495)

code:

#include<iostream>
#define size 10000
using namespace std;
int max_fibo=0;
string fibo_n[5001];
string big_sum(string s1,string s2){
    int l1,l2,carry=0;
    int sum,i=0;
    char result[size],ch;
    for(l1=0;s1[l1]!='\0';l1++);
    for(l2=0;s2[l2]!='\0';l2++);
    for(l1--,l2--;l1>=0;l1--,l2--){
        if(l2>=0){
            sum=(s1[l1]-'0')+(s2[l2]-'0')+carry;
        }else sum=(s1[l1]-'0')+carry;
        carry=sum/10;
        result[i++]=(sum%10)+'0';
    }if(carry) result[i++]=carry+'0';
    result[i]='\0';
    for(l2=0;result[l2]!='\0';l2++);
    l1=0;
    for(l2--;l1<l2;l2--,l1++){
        ch=result[l2];
        result[l2]=result[l1];
        result[l1]=ch;
    }
    return result;
}
int main()
{
    fibo_n[0]="0";
    fibo_n[1]="1";
    for(int i=2;i<=5000;i++){
        fibo_n[i]=big_sum(fibo_n[i-1],fibo_n[i-2]);
    }
    int N;
    while(cin>>N){
        cout<<"The Fibonacci number for "<<N<<" is "<<fibo_n[N]<<endl;
    }return 0;
}

NB: Here first number greater than second number.

problem link:Fibonacci Freeze