L. Right Build bfs

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
#define IOS ios::sync_with_stdio(false)
using namespace std;
#define inf (0x3f3f3f3f)
typedef long long int LL;

#include <iostream>
#include <sstream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <string>

const int maxn = 5e5 + 20;
struct node {
    int u, v, tonext;
} e[maxn];
int num;
int first[maxn];
int used;
void add(int u, int v) {
    ++num;
    e[num].u = u;
    e[num].v = v;
    e[num].tonext = first[u];
    first[u] = num;
}
int n, m, a, b;
int ans;
int book[maxn];
int zero[maxn];
struct bfsnode {
    int cur;
    int cnt;
    bfsnode(int a, int b) : cur(a), cnt(b) {}
};
queue<struct bfsnode>que, quese;
int bfs(int begin, int end, int DFN) {
    if (begin == end) return 0;
    book[begin] = DFN;
    while (!quese.empty())  quese.pop();
    quese.push(bfsnode(begin, 0));
    while (!quese.empty()) {
        struct bfsnode t = quese.front();
        quese.pop();
        for (int i = first[t.cur]; i; i = e[i].tonext) {
            int v = e[i].v;
            if (book[v] == DFN) continue;
            if (v == end) return t.cnt + 1;
            zero[v] = t.cnt + 1;
            book[v] = DFN;
            quese.push(bfsnode(v, t.cnt + 1));
        }
    }
    return inf;
}
int first_two[maxn];
struct Edge {
    int u, v, tonext;
} e_two[maxn];
int num_two;
void add_two(int u, int v) {
    ++num_two;
    e_two[num_two].u = u;
    e_two[num_two].v = v;
    e_two[num_two].tonext = first_two[u];
    first_two[u] = num_two;
}
int bookA[2][maxn];
int book_two[maxn];
void BFS(int begin, int DFN, int now) {
    while (!que.empty()) que.pop();
    book_two[begin] = DFN;
    bookA[now][begin] = 0;
    if (bookA[!now][begin] != inf && zero[begin] != inf) {
        ans = min(bookA[now][begin] + bookA[!now][begin] + zero[begin], ans);
    }
    que.push(bfsnode(begin, 0));
    while (!que.empty()) {
        struct bfsnode t = que.front();
        que.pop();
        for (int i = first_two[t.cur]; i; i = e_two[i].tonext) {
            int v = e_two[i].v;
            if (book_two[v] == DFN) continue;
            book_two[v] = DFN;
            bookA[now][v] = t.cnt + 1;
            if (bookA[!now][v] != inf && zero[v] != inf) {
                ++used;
                int tans = zero[v] + bookA[now][v] + bookA[!now][v];
                ans = min(ans, tans);
            }
            que.push(bfsnode(v, t.cnt + 1));
        }
    }
    return;
}
void find() {
    memset(bookA, 0x3f, sizeof bookA);
    ans = inf;
    used = 1;
    BFS(a, used, 0);
    ++used;
    BFS(b, used, 1);
    return;
}

int in[maxn];
void work() {
    IOS;
    cin >> n >> m >> a >> b;
    if (a == b) while(1);
    for (int i = 1; i <= m; ++i) {
        int u, v;
        cin >> u >> v;
        add(u, v);
        add_two(v, u);
    }
    memset(zero, 0x3f, sizeof zero);
    zero[0] = 0;
    bfs(0, inf, 8);
    find();
    cout << ans << endl;
}
int main() {
#ifdef local
    freopen("data.txt","r",stdin);
#endif
    work();
    return 0;
}