Sum two double numbers with increased accuracy

#include <iostream>
#include <iomanip>
#include <string>
#include <cmath>

using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    string str;
    long long f = 0, q = 0;
    int fl = 0, ql = 0;

    while (getline(cin, str)) {
        int i = 0;
        string tmp;
        for (; str[i] != '.'; ++i) {
            tmp = tmp + str[i];
        }
        i++;
        f += stoll(tmp);
        tmp = "";
        for (; i < str.length(); ++i) {
            tmp = tmp + str[i];
        }
        q += stoll(tmp);
    }
    fl = to_string(f).length();
    f += q / pow(10, 15);
    q = q % (long long)pow(10, 15);
    ql = to_string(q).length();

    cout << f << ".";
    for (int j = 0; j < 15 - ql; ++j) {
        cout << 0;
    }
    cout << q;
    //cout << setprecision(15);

    return 0;
}