Uva 11743 - Credit Check

#include<iostream>
#include<cmath>
#include<string>
#include<cstdio>
#include<cstring>

using namespace std;

typedef long long int LL;

LL digit_sum(LL n)
{
    LL sum=0;

    while(n)
    {
        sum+=(n%10);
        n/=10;
    }

    return sum;
}

bool isCheck(LL n)
{
    return n%10;
}

int main()
{
    int t;
    cin >> t;
    getchar();
    string s;
    while(t--)
    {
        getline(cin,s);

        LL len = s.size(),i,ds=0;

        for(i=0;i<len;i+=2)
        {
            if(s[i]==' ')i++;
            ds+=(digit_sum((s[i]-48)*2))+(s[i+1] - 48);
        }

        //cout << ds << endl;
        if(!isCheck(ds))
        {
            cout << "Valid" << endl;
        }
        else
        {
            cout << "Invalid" << endl;
        }
    }

    return 0;
}