#include <iostream>
#include <vector>
using namespace std;

long  sumBigNumber(_______)
{
    _______
}
int main()
{
    string numA, numB;
    cin >> numA >> numB;
    cout << sumBigNumber(numA, numB);
    return 0;
}

Line 5: error: '_______' was not declared in this scope
compilation terminated due to -Wfatal-errors.