#include<bits/stdc++.h>

#define int long long
#define pb push_back
#define _ << ' ' <<
#define end << "\n"

const int mod = 1e9 + 7;
typedef long long ll;
typedef long double ld;

using namespace std;

void solve(){
    int n;
    cin >> n;
    string a;
    cin >> a;
    vector<int> ans;
    int i = 1;
    while(i < n){
        if(a[i] == a[i - 1]){
            if(i > 1 && a[i - 2] != a[i]){
                if(a[i] == 'W'){
                    a[i] = 'B';
                    a[i - 1] = 'B';
                }
                else{
                    a[i] = 'W';
                    a[i - 1] = 'W';
                }
                ans.push_back(i - 1);
            }
            i += 2;
        }
        else{
            if(i > 1 && a[i - 2] != a[i - 1]){
                if(a[i] == 'W'){
                    a[i] = 'B';
                    a[i - 1] = 'W';
                }
                else{
                    a[i] = 'W';
                    a[i - 1] = 'B';
                }
                ans.push_back(i - 1);
            }
            else if(i + 1 < n && a[i + 1] != a[i]){
                if(a[i] == 'W'){
                    a[i] = 'B';
                    a[i - 1] = 'W';
                }
                else{
                    a[i] = 'W';
                    a[i - 1] = 'B';
                }
                ans.push_back(i - 1);
            }
            i += 1;
        }
    }
    if(a[n - 1] != a[n - 2]){
        cout << -1;
    }
    else{
        cout << ans.size() << '\n';
        for(auto x: ans){
            cout << x + 1 << ' ';
        }
    }
}
signed main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int we = 1;
    //cin >> we;
    while(we--){
        solve();
    }
}