#pragma GCC optimize("Ofast")
#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <cstdio>
#include <ctime>
#include <assert.h>
#include <chrono>
#include <random>
#include <numeric>
#include <set>
#include <deque>
#include <stack>
#include <sstream>
#include <utility>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n; cin >> n;
    string s; cin >> s;
    int res = 0;
    for (int _ = 0; _ < 2; ++_){
        int cnt = 0;
        for(int j=0;j<n;j++){
            if(cnt == 0){
                if(j>0 and s[j-1] == 'W' and s[j] == 'B'){
                    cnt++;
                }
            }
            else{
                if(s[j]=='B')cnt++;
                else if(s[j]=='W'){
                    cnt = 0;
                }
                else{
                    bool ok=true;
                    j++;
                    while(j<n){
                        if(s[j] == 'W')j++;
                        else if(s[j] == '.'){
                            break;
                        }
                        else{
                            ok=false;
                            break;
                        }
                    }
                    j--;
                    if(ok) res = max(res,cnt);
                    cnt = 0;
                }
            }
        }
        reverse(s.begin(), s.end());
    }
    cout << res << endl;
}