String multiplication (Big Int)

For input =1,2,3,4,5
Output =120
1*2*3*4*5=120


///...................SUBHASHIS MOLLICK...................///
///.....DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING....///
///.............ISLAMIC UNIVERSITY,BANGLADESH.............///
///....................SESSION-(14-15)....................///
#include<bits/stdc++.h>
using namespace std;
#define sf(a) scanf("%lld",&a)
#define sf2(a,b) scanf("%lld %lld",&a,&b)
#define sf3(a,b,c) scanf("%lld %lld %lld",&a,&b,&c)
#define pf(a) printf("%lld",a)
#define pf2(a,b) printf("%lld %lld",a,b)
#define pf3(a,b,c) printf("%lld %lld %lld",a,b,c)
#define nl printf("\n")
#define ll long long
#define pb push_back
#define MPI map<int,int>mp;
#define fr(i,n) for(i=0;i<n;i++)
#define fr1(i,n) for(i=1;i<=n;i++)
#define frl(i,a,b) for(i=a;i<=b;i++)
//freopen("Input.txt","r",stdin);
//freopen("Output.txt","w",stdout);
//const int fx[]={+1,-1,+0,+0};
//const int fy[]={+0,+0,+1,-1};
//const int fx[]={+0,+0,+1,-1,-1,+1,-1,+1};   // Kings Move
//const int fy[]={-1,+1,+0,+0,+1,+1,-1,-1};  // Kings Move
//const int fx[]={-2, -2, -1, -1,  1,  1,  2,  2};  // Knights Move
//const int fy[]={-1,  1, -2,  2, -2,  2, -1,  1}; // Knights Move
string multiple(string fst,long b)
{
    long carry = 0;

    for( long i = 0; i < fst.size(); i++ )
    {
        carry += (fst[i] - 48) * b;
        fst[i] = ( carry % 10 + 48 );
        carry /= 10;
    }

    while( carry )
    {
        fst += ( carry % 10 + 48 );
        carry /= 10;
    }
    return fst;
}
main()
{
    string ans;
    long i,a;
    for(i=1; i<=5; i++)
    {
        cin>>a;
        if(i==1) ///prothom bar er jonno alada kore jog
        {
            while(a!=0)
            {
                ans+=(a%10)+'0';
                a/=10;
            }
        }
        else     ///baki man gulor jonno alada jog hobe
        ans=multiple(ans,a);
    }
    reverse(ans.begin(),ans.end());
    cout<<ans<<endl;
}