Declaring lambda with int type not working [closed]

I wanted to use the lambda function somehow, but it doesn't work, and I don't know why.

vector<int> v;

/* ... */

int median = [](vector<int> a)
{
    sort(a.begin(), a.end());
    return a[a.size() / 2];
} 

I created a lambda function, but how can I call it? int median = [](vector<int> a) { ... } (v)? But it doesn't work too.

I have seen lambda functions only in some examples like making custom comparator. Can I make it work in my case somehow?