It's only 2 lines even restricting yourself to the standard library

    #include <fstream>
    #include <iterator>
    #include <numeric>
    using namespace std;

    int main() {
        ifstream nf ("numbers.txt");
        return accumulate (istream_iterator<int>(nf), istream_iterator<int>(), 0);
    }

Thank you for this. Got me really interested in folly now.