#include <fstream>
#include <vector>
#include <bitset>
#include <queue>
#include <algorithm>
#include <utility>
#include <cstring>
#include <string>
#include <stack>
#include <deque>
#include <iomanip>
#include <set>
#include <map>
#include <cassert>
#include <ctime>
#include <list>
#include <iomanip>

using namespace std;

string file = "joc4";

ifstream cin( (file + ".in").c_str() );
ofstream cout( (file + ".out").c_str() );

const int MAXN = 2*255;
const int oo = 10000;

typedef vector<short> Graph[MAXN];
typedef vector<short> :: iterator It;

Graph G;
short N, M, A, B, C[MAXN][MAXN], Father[MAXN], maxFlow;
bitset <MAXN> Used;
queue <short> Q;

inline bool BFs() {
    Used.reset();
    for(Q.push(A), Used[A] = true ; !Q.empty() ; Q.pop()) {
        short Node = Q.front();
        for(It it = G[Node].begin(), fin = G[Node].end(); it != fin ; ++ it)
            if(!Used[*it] && C[Node][*it] > 0)
                Q.push(*it), Used[*it] = true, Father[*it] = Node;
    }
    return Used[B+N];
}

int main() {
    cin >> N >> M >> A >> B;
    for(int i = 1 ; i <= M ; ++ i) {
        short x, y;
        cin >> x >> y;
        G[x+N].push_back(y);
        G[y].push_back(x+N);
        C[x+N][y] = 1;
        C[y][x+N] = 1;
    }
    for(int i = 1 ; i <= N ; ++ i)
        G[i].push_back(N+i), C[i][N+i] = 1;
    G[A].push_back(N+A);
    G[B].push_back(N+B);
    G[N+B].push_back(B);
    C[A][N+A] = oo;
    C[N+B][B] = oo;
    C[B][N+B] = oo;
    for(maxFlow = 0 ; BFs() ; ) {
        for(It it = G[B+N].begin(), fin = G[B+N].end(); it != fin ; ++ it) {
            if(C[*it][B+N] <= 0 || !Used[*it])
                continue;
            Father[B+N] = *it;
            for(short i = B + N ; i != A ; i = Father[i])
                C[Father[i]][i] --;
            maxFlow ++;
        }
    }
    cout << maxFlow << '\n';
    cin.close();
    cout.close();
    return 0;
}