import std.stdio;
import std.string;
import std.format;
import std.conv;
import std.typecons;
import std.algorithm;
import std.functional;
import std.bigint;
import std.numeric;
import std.array;
import std.math;
import std.range;
import std.container;
import std.concurrency;
import std.traits;
import std.uni;
import core.bitop : popcnt;
alias Generator = std.concurrency.Generator;

enum long INF = long.max/5;
enum long MOD = 10L^^9+7;

class Vertex {
  long index;
  Vertex[] adj;
  Vertex targetV;
  Vertex nextV;
  Vertex furthestV;
  long dist;
  long furthestD;

  mixin Constructor;
}

void main() {
  long N, s, g;
  scanln(N, s, g);
  s--; g--;

  Vertex[] vs = N.iota.map!(i => new Vertex(i)).array;
  foreach(_; 0..N-1) {
    long a, b;
    scanln(a, b);
    a--; b--;
    vs[a].adj ~= vs[b];
    vs[b].adj ~= vs[a];
  }

  vs.each!(v => v.dist = INF);
  vs[s].dist = 0;
  {
    auto list = DList!Vertex(vs[s]);
    while(!list.empty) {
      Vertex v = list.front;
      list.removeFront;
      foreach(u; v.adj) {
        long d = v.dist + 1;
        if (d < u.dist) {
          u.dist = d;
          u.nextV = v;
          list.insertBack(u);
        }
      }
    }
  }

  void f(Vertex v, Vertex fromV = null) {
    v.furthestD = 0;
    v.furthestV = v;
    foreach(u; v.adj) {
      if (u is fromV) continue;
      f(u, v);
      if (u.furthestD + 1 > v.furthestD) {
        v.furthestD = u.furthestD + 1;
        v.furthestV = u.furthestV;
      }
    }
  }
  f(vs[g]);

  long ans = vs[s].furthestD + vs[g].dist - 1;

  Vertex v = vs[g];
  Vertex u = vs[s];
  foreach(i; 0..N) {
    if (v.nextV is null) break;

    Vertex w = v.nextV.nextV;
    if (w !is null && u !is w.furthestV) {
      assert(u is vs[s]);
      if (w.dist <= i + 1) {
        u = w.furthestV;
        ans.ch!max(i + 2 + w.furthestD - 1);
        break;
      }
    }
    v = v.nextV;
  }

  ans.writeln;
}

// ----------------------------------------------


void times(alias fun)(long n) {
  // n.iota.each!(i => fun());
  foreach(i; 0..n) fun();
}
auto rep(alias fun, T = typeof(fun()))(long n) {
  // return n.iota.map!(i => fun()).array;
  T[] res = new T[n];
  foreach(ref e; res) e = fun();
  return res;
}

T ceil(T)(T x, T y) if (isIntegral!T || is(T == BigInt)) {
  // `(x+y-1)/y` will only work for positive numbers ...
  T t = x / y;
  if (y > 0 && t * y < x) t++;
  if (y < 0 && t * y > x) t++;
  return t;
}

T floor(T)(T x, T y) if (isIntegral!T || is(T == BigInt)) {
  T t = x / y;
  if (y > 0 && t * y > x) t--;
  if (y < 0 && t * y < x) t--;
  return t;
}

ref T ch(alias fun, T, S...)(ref T lhs, S rhs) {
  return lhs = fun(lhs, rhs);
}
unittest {
  long x = 1000;
  x.ch!min(2000);
  assert(x == 1000);
  x.ch!min(3, 2, 1);
  assert(x == 1);
  x.ch!max(100).ch!min(1000); // clamp
  assert(x == 100);
  x.ch!max(0).ch!min(10); // clamp
  assert(x == 10);
}

mixin template Constructor() {
  import std.traits : FieldNameTuple;
  this(Args...)(Args args) {
    // static foreach(i, v; args) {
    foreach(i, v; args) {
      mixin("this." ~ FieldNameTuple!(typeof(this))[i]) = v;
    }
  }
}

void scanln(Args...)(auto ref Args args) {
  enum sep = " ";
  enum n = Args.length;
  enum fmt = n.rep!(()=>"%s").join(sep) ~ "\n";
  static if (__VERSION__ >= 2071) {
    readf!fmt(args);
  } else {
    enum argsTemp = n.iota.map!(
      i => "&args[%d]".format(i)
    ).join(", ");
    mixin(
      "readf(fmt, " ~ argsTemp ~ ");"
    );
  }
}

// fold was added in D 2.071.0
static if (__VERSION__ < 2071) {
  template fold(fun...) if (fun.length >= 1) {
    auto fold(R, S...)(R r, S seed) {
      static if (S.length < 2) {
        return reduce!fun(seed, r);
      } else {
        return reduce!fun(tuple(seed), r);
      }
    }
  }
}

// popcnt with ulongs was added in D 2.071.0
static if (__VERSION__ < 2071) {
  ulong popcnt(ulong x) {
    x = (x & 0x5555555555555555L) + (x>> 1 & 0x5555555555555555L);
    x = (x & 0x3333333333333333L) + (x>> 2 & 0x3333333333333333L);
    x = (x & 0x0f0f0f0f0f0f0f0fL) + (x>> 4 & 0x0f0f0f0f0f0f0f0fL);
    x = (x & 0x00ff00ff00ff00ffL) + (x>> 8 & 0x00ff00ff00ff00ffL);
    x = (x & 0x0000ffff0000ffffL) + (x>>16 & 0x0000ffff0000ffffL);
    x = (x & 0x00000000ffffffffL) + (x>>32 & 0x00000000ffffffffL);
    return x;
  }
}


Measured memory_bytes: 28988