abc174_d - Alter Altar

using namespace std;
int solve(int n, string &s)
{
    int l, r, ans = 0;
    l = 0, r = n - 1;
    while (l < r)
    {
        while (l < n and s[l] != 'W')
            ++l;
        while (r >= 0 and s[r] != 'R')
            --r;
        if (l >= r)
            break;
        swap(s[l], s[r]);
        ++ans;
    }
    return ans;
}
int main(void)
{
    int n;
    string s;
    cin >> n;
    cin >> s;
    cout << solve(n, s) << endl;
    return 0;
}