Code

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>

using namespace std;
typedef vector<int> lnum;
const int base = 1000*1000*1000;

int main()
{
   lnum a,b;
    string s;
    cin>>s;
for (int i=(int)s.length(); i>0; i-=9)
    if (i < 9)
        a.push_back (atoi (s.substr (0, i).c_str()));
    else
        a.push_back (atoi (s.substr (i-9, 9).c_str()));

    cin>>s;
        for (int i=(int)s.length(); i>0; i-=9)
    if (i < 9)
        b.push_back (atoi (s.substr (0, i).c_str()));
    else
        b.push_back (atoi (s.substr (i-9, 9).c_str()));

lnum c (a.size()+b.size());
for (size_t i=0; i<a.size(); ++i)
    for (int j=0, carry=0; j<(int)b.size() || carry; ++j) {
        long long cur = c[i+j] + a[i] * 1ll * (j < (int)b.size() ? b[j] : 0) + carry;
        c[i+j] = int (cur % base);
        carry = int (cur / base);
    }
while (c.size() > 1 && c.back() == 0)
    c.pop_back();

printf ("%d", c.empty() ? 0 : c.back());
for (int i=(int)c.size()-2; i>=0; --i)
    printf ("%09d", c[i]);
    return 0;
}