#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using namespace std;

typedef long long ll;

const int MAX_P = 3;
int fact[MAX_P];

int extgcd(int a, int b, int& x, int& y) {
    int d = a;
    if (b != 0) {
        d = extgcd(b, a % b, y, x);
        y -= (a / b) * x;
    } else {
        x = 1; y = 0;
    }
    return d;
}

int mod_inverse(int a, int m) {
    int x, y;
    extgcd(a, m, x, y);
    return (m + x % m) % m;
}

// n!=a p^e としたときのa mod pを求める. O(p)
int mod_fact(int n, int p, int&e) {
    e = 0;
    if (n == 0) return 1;

    // pの倍数の部分を計算
    int res = mod_fact(n / p, p, e);
    e += n / p;

    // (p-1)!=-1なので(p-1)!^(n/p)はn/pの偶奇だけで計算できる
    if (n / p % 2 != 0) return res * (p - fact[n % p]) % p;
    return res * fact[n % p] % p;
}

// nCk mod pを求める. O(log_p n)
int mod_comb(int n, int k, int p) {
    if (n < 0 || k < 0 || n < k) return 0;
    int e1, e2, e3;
    int a1 = mod_fact(n, p, e1), a2 = mod_fact(k, p, e2), a3 = mod_fact(n - k, p, e3);
    if (e1 > e2 + e3) return 0;
    return a1 * mod_inverse(a2 * a3 % p, p) % p;
}

int main() {
    fact[0] = 1;
    fact[1] = 1;
    fact[2] = 2;
    int N;
    string c;
    cin >> N >> c;
    int s = 0;
    rep(i, N) {
        int x;
        if (c[i] == 'B') x = 0;
        if (c[i] == 'W') x = 1;
        if (c[i] == 'R') x = 2;
        s = (s + x * (2 - (N % 2)) * mod_comb(N - 1, i, 3)) % 3;
        //cout << mod_comb(N - 1, i, 3) << " " << s << endl;
    }
    if (s == 0) printf("B\n");
    if (s == 1) printf("W\n");
    if (s == 2) printf("R\n");
}
#include "bits/stdc++.h"
using namespace std;
#define int               long long
#define pb                push_back
#define ppb               pop_back
#define pf                push_front
#define ppf               pop_front
#define all(x)            (x).begin(),(x).end()
#define uniq(v)           (v).erase(unique(all(v)),(v).end())
#define sz(x)             (int)((x).size())
#define fr                first
#define sc                second
#define pii               pair<int,int>
#define rep(i,a,b)        for(int i=a;i<b;i++)
#define mem1(a)           memset(a,-1,sizeof(a))
#define mem0(a)           memset(a,0,sizeof(a))
#define ppc               __builtin_popcount
#define ppcll             __builtin_popcountll

template<typename T1,typename T2>istream& operator>>(istream& in,pair<T1,T2> &a){in>>a.fr>>a.sc;return in;}
template<typename T1,typename T2>ostream& operator<<(ostream& out,pair<T1,T2> a){out<<a.fr<<" "<<a.sc;return out;}
template<typename T,typename T1>T amax(T &a,T1 b){if(b>a)a=b;return a;}
template<typename T,typename T1>T amin(T &a,T1 b){if(b<a)a=b;return a;}

const long long INF=1e18;
const int32_t M=1e9+7;
const int32_t MM=998244353;

const int N=0;



void solve(){
	int a,b,c,d;
	cin >> a >> b >> c >> d;
	cout << a * d - b * c;
}
signed main(){
	ios_base::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	//freopen("input.txt", "r", stdin);
	//freopen("output.txt", "w", stdout);
	#ifdef SIEVE
		sieve();
	#endif
	#ifdef NCR
		init();
	#endif
	int t=1;
	//cin>>t;
	while(t--) solve();
	return 0;
}
