how to merge more than one hashmaps also sum the values of same key in java

ı am trying to merge more than one hashmaps also sum the values of same key, ı want to explain my problem with toy example as follows

    HashMap<String, Integer> m = new HashMap<>();
    HashMap<String, Integer> m2 = new HashMap<>();

    m.put("apple", 2);
    m.put("pear", 3);
    m2.put("apple", 9);
    m2.put("banana", 6);

ı tried putall

m.putAll(m2);

output is as follows {banana=6, apple=9, pear=3}

but its result is not true for this problem. ı want to output as

{banana=6, apple=11, pear=3}

how can ı get this result in java?