#include <iostream>
#include <queue>
#include <vector>

#define c_boost std::ios_base::sync_with_stdio(false); std::cin.tie(nullptr)

using namespace std;
using graph=vector <vector <int>>;

vector <bool> used;
vector <int> reverseEdges;

graph g;
graph gT;

void BFS(int start) {
    queue <int> q;
    q.push(start);
    while (!q.empty()) {
        int from = q.front();
        q.pop();
        used[from] = true;
        for (int to: g[from]) {
            reverseEdges[to] = min(reverseEdges[to], reverseEdges[from]);
            if (!used[to]) {
                q.push(to);
            }
        }
        for (int backTo : gT[from]) {
            reverseEdges[backTo] = min(reverseEdges[backTo], reverseEdges[from] + 1);
            if (!used[backTo]) {
                q.push(backTo);
            }
        }
    }
}

int main() {
    c_boost;
    int n, m, start, end;
    cout << "Введите {n}, {m}, {start}, {end}\n";
    cin >> n >> m >> start >> end;
    used.assign(n, false);
    reverseEdges.assign(n, 0);
    g = graph(n);
    gT = graph(n);
    for (int i = 0; i < m; ++i) {
        int from, to;
        cin >> from >> to;
        --from;
        --to;
        g[from].push_back(to);
        gT[to].push_back(from);
    }
    BFS(start - 1);
    cout << reverseEdges[end - 1];
}