#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double lf;
typedef pair<int,int> ii;
typedef pair<ii,int> iii;
#define REP(i,n) for(int i=0;i<n;i++)
#define REP1(i,n) for(int i=1;i<=n;i++)
#define RST(i,n) memset(i,n,sizeof i)
#define SZ(a) (int)a.size()
#define ALL(a) a.begin(),a.end()
#define X first
#define Y second
#define eb emplace_back
#ifdef cold66
#define debug(...) do{\
    fprintf(stderr,"LINE %d: (%s) = ",__LINE__,#__VA_ARGS__);\
    _do(__VA_ARGS__);\
}while(0)
template<typename T>void _do(T &&_x){cerr<<_x<<endl;}
template<typename T,typename ...S> void _do(T &&_x,S &&..._t){cerr<<_x<<", ";_do(_t...);}
template<typename _a,typename _b> ostream& operator << (ostream &_s,const pair<_a,_b> &_p){return _s<<"("<<_p.X<<","<<_p.Y<<")";}
template<typename It> ostream& _OUTC(ostream &_s,It _ita,It _itb)
{
    _s<<"{";
    for(It _it=_ita;_it!=_itb;_it++)
    {
        _s<<(_it==_ita?"":",")<<*_it;
    }
    _s<<"}";
    return _s;
}
template<typename _a> ostream &operator << (ostream &_s,vector<_a> &_c){return _OUTC(_s,ALL(_c));}
template<typename _a> ostream &operator << (ostream &_s,set<_a> &_c){return _OUTC(_s,ALL(_c));}
template<typename _a,typename _b> ostream &operator << (ostream &_s,map<_a,_b> &_c){return _OUTC(_s,ALL(_c));}
template<typename _t> void pary(_t _a,_t _b){_OUTC(cerr,_a,_b);cerr<<endl;}
#define IOS()
#else
#define debug(...)
#define pary(...)
#define endl '\n'
#define IOS() ios_base::sync_with_stdio(0);cin.tie(0);
#endif // cold66
//}
template<class T> inline bool chkmax(T &a, const T &b) { return b > a ? a = b, true : false; }
template<class T> inline bool chkmin(T &a, const T &b) { return b < a ? a = b, true : false; }

const ll MAXn=2e5+5,MAXlg=__lg(MAXn)+2;
const ll MOD=1000000007;
const ll INF=0x3f3f3f3f;

int n,X,Y;
int ans = 0;
vector<int> r[MAXn],b[MAXn];
int ancb[MAXlg][MAXn],depb[MAXn];
void dfsb(int x,int p){
    if (x != p) depb[x] = depb[p] + 1;
    debug(x,depb[x]);
    ancb[0][x] = p;
    for (int i=1;1<<i < MAXn;++i) {
        ancb[i][x] = ancb[i-1][ancb[i-1][x]];
    }
    for (auto i:b[x]) {
        if (i == p) continue;
        dfsb(i,x);
    }
}
int LCA(int x,int y){
    if (depb[x] < depb[y]) {
        swap(x,y);
    }
    for (int i=MAXlg-1;i>=0;--i) {
        if (((depb[x] - depb[y])>>i)&1) {
            x = ancb[i][x];
        }
    }
    if (x == y) return x;
    for (int i=MAXlg-1;i>=0;--i) {
        if (ancb[i][x] != ancb[i][y]) {
            x = ancb[i][x];
            y = ancb[i][y];
        }
    }
    return ancb[0][x];
}
int dis(int x,int y){
    int lca = LCA(x,y);
    debug(x,y,lca);
    return depb[x] + depb[y] - 2 * depb[lca];
}
void dfsr(int x,int p,int d){
    debug(x,p,d,depb[x]);
    ans = max(ans,depb[x] * 2);
    for (auto i:r[x]) {
        if (i == p) continue;
        if (dis(x,i) >= 3) {
            debug(x,i);
            ans = INF;
            break;
        }
        if (d + 1 < depb[i]) dfsr(i,x,d+1);
        else if (d + 1 == depb[i]) ans = max(ans,depb[i] * 2);
    }
}
int main(){
    IOS();
    cin >> n >> X >> Y;
    for (int i=0;i<n-1;++i) {
        int u,v;
        cin >> u >> v;
        r[u].eb(v);
        r[v].eb(u);
    }
    for (int i=0;i<n-1;++i) {
        int u,v;
        cin >> u >> v;
        b[u].eb(v);
        b[v].eb(u);
    }
    dfsb(Y,Y);
    dfsr(X,X,0);
    if (ans == INF) cout << -1 << endl;
    else cout << ans << endl;
}


Measured memory_bytes: 50044