O(N+M) time complexity

A simple example of algorithm that is O(m+n):

int sum(int[] nArr, int[] mArr) {
    int sum = 0;
    for(int i : nArr) {
        sum += i;
    }
    for(int i : mArr) {
        sum += i;
    }
    return sum;
}

To compute the sum, you need to go through all elements in nArr (size n) and all elements in mArr (size m), so the overall complexity is O(m+n)