#include<bits/stdc++.h>
using namespace std;
int nr, N, mouse, trap, t[1000009], dp[1000009], sda[1000009], deg[1000009], node[1000009];
vector < int > v[1000009], h[1000009];
void dfs (int nod)
{
sda[nod] = (nod == trap ? 0 : sda[t[nod]] - 1 + (int) (v[nod].size ()) - 1);
for (auto it : v[nod])
if (it != t[nod])
t[it] = nod, dfs (it), deg[nod] ++;
if (deg[nod] <= 1) dp[nod] = deg[nod];
else
{
int max1 = -N, max2 = -N;
for (auto it : v[nod])
if (it != t[nod])
{
if (dp[it] > max1) max2 = max1, max1 = dp[it];
else
if (dp[it] > max2) max2 = dp[it];
}
dp[nod] = deg[nod] + max2;
}
}
int getDp (int i, int cnt)
{
if (cnt >= h[i].size ()) return 0;
return h[i][cnt];
}
bool ok (int D)
{
int minS = 0;
for (int i=1; i<nr; i++)
{
int oldS = minS;
while (minS <= i && max (0, sda[t[node[i]]] - (i - minS)) + getDp (i, minS - oldS) +
max (0, deg[node[i]] - (i > 1) - (minS - oldS)) + min (i, sda[t[node[i]]] + minS) > D)
minS ++;
if (minS > i) return 0;
}
return 1;
}
int main ()
{
//freopen ("input", "r", stdin);
//freopen ("output", "w", stdout);
scanf ("%d %d %d", &N, &trap, &mouse);
for (int i=1; i<N; i++)
{
int x, y;
scanf ("%d %d", &x, &y);
v[x].push_back (y);
v[y].push_back (x);
}
dfs (trap);
node[1] = mouse, nr = 1;
while (node[nr] != trap)
node[nr + 1] = t[node[nr]], nr ++;
for (int i=1; i<=nr; i++)
{
for (auto j : v[node[i]])
if (j != t[node[i]] && j != node[i - 1])
h[i].push_back (dp[j]);
sort (h[i].begin (), h[i].end ());
reverse (h[i].begin (), h[i].end ());
}
int p = 0, u = N, mij, ras = -1;
while (p <= u)
{
mij = (p + u) >> 1;
if (ok (mij)) ras = mij, u = mij - 1;
else p = mij + 1;
}
printf ("%d\n", ras);
return 0;
}
