#include <iostream>
#include <unordered_set>
#include <limits>

int symmetricDifferenceCount(int sizeA, int sizeB, const std::string& setA, const std::string& setB) {
    std::unordered_set<char> setA_chars(setA.begin(), setA.end());
    std::unordered_set<char> setB_chars(setB.begin(), setB.end());

    std::unordered_set<char> symmetricDifference;

    for (char element : setA_chars) {
        if (setB_chars.find(element) == setB_chars.end()) {
            symmetricDifference.insert(element);
        }
    }

    for (char element : setB_chars) {
        if (setA_chars.find(element) == setA_chars.end()) {
            symmetricDifference.insert(element);
        }
    }

    return symmetricDifference.size();
}

int main() {
    int sizeA, sizeB;



    std::cin >> sizeA;

    std::cin >> sizeB;


    if (sizeA < 0 || sizeB < 0 || sizeA > 20 || sizeB > 20) {
        std::cout << "Invalid input data!" << std::endl;
        return 0;
    }


    std::string setA, setB;

    std::cin >> setA;


    std::cin >> setB;


    int result = symmetricDifferenceCount(sizeA, sizeB, setA, setB);
    std::cout << result << std::endl;

    return 0;
}