#include <bits/stdc++.h>
#define all(x) (x).begin(), (x).end()
#define FastIO ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
using namespace std;
typedef long long ll;
void bfs(vector<ll> &mind, const ll sv, const vector<vector<ll>> &graph) {
queue<array<ll, 2>> q;
q.push({ sv, 0 });
while(!q.empty()) {
auto [v, d] = q.front();
q.pop();
if(mind[v] != -1)
continue;
mind[v] = d;
for(const auto &x : graph[v])
q.push({ x, d + 1 });
}
}
int main() {
FastIO;
ll i, u, v, ans1, N, M, A, B;
cin >> N >> M >> A >> B;
vector<vector<ll>> graph(N + 1);
for(i = 0; i < M; i++) {
cin >> u >> v;
graph[u].push_back(v);
graph[v].push_back(u);
}
vector<ll> minda(N + 1, -1);
vector<ll> mindb(N + 1, -1);
bfs(minda, A, graph);
bfs(mindb, B, graph);
ans1 = 0;
vector<bool> ans2(N + 1);
array<ll, 2> cntr = { 0, 0 };
for(i = 1; i <= N; i++) {
ans1 += min(minda[i], mindb[i]);
ans2[i] = (minda[i] < mindb[i]);
cntr[minda[i] < mindb[i]]++;
}
vector<array<ll, 2>> diffs;
for(i = 1; i <= N; i++) {
if(ans2[i] == max_element(all(cntr)) - cntr.begin())
diffs.push_back({ abs(minda[i] - mindb[i]), i });
}
sort(all(diffs));
for(i = 0; i < abs(cntr[0] - cntr[1]) / 2; i++) {
ans1 += diffs[i][0];
ans2[diffs[i][1]] = !ans2[diffs[i][1]];
}
cout << ans1 << '\n';
for(i = 1; i <= N; i++)
cout << (ans2[i] ? 'A' : 'B');
cout << '\n';
return 0;
}