#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=1e5+10;
typedef pair<int,int> pii;
vector<int> edge[N];
int n,m;
int w[N];
map<pii,int> id;
bool dfs(int s,int u,int father,int v)
{
    if(u==v) return true;
    for(int i=0;i<edge[u].size();i++)
    {
        int son=edge[u][i];
        if(son==father) continue;
        if(dfs(s,son,u,v))
        {
            int ID=id[{u,son}];
            w[ID]++;
            return true;
        }
    }
    return false;
}
signed main()
{
    cin>>n>>m;
    for(int i=1;i<n;i++)
    {
        int x,y; cin>>x>>y;
        edge[x].push_back(y);
        edge[y].push_back(x);
        id[{x,y}]=id[{y,x}]=i;
    }
    for(int i=0;i<m;i++)
    {
        int x,y; cin>>x>>y;
        dfs(x,x,-1,y);
    }
    int ans=-1;
    for(int i=n-1;i>=1;i--)
    {
        if(w[i]==m)
        {
            ans=i;
            break;
        }
    }
    cout<<ans;
    return 0;
}

#include<bits/stdc++.h>
#define int long long
using namespace std;
typedef pair<int,int>pii;
const int N=1e5+10;
int n,m;
int w[N];
map<pii,int>id;
vector<int>edge[N];
int dep[N],fa[N][20];
void dfs(int u,int father)
{
    dep[u]=dep[father]+1;
    fa[u][0]=father;
    for(int i=1;i<=19;i++)
    {
        fa[u][i]=fa[fa[u][i-1]][i-1];
    }
    for(int v:edge[u])
    {
        if(v!=father) dfs(v,u);
    }
}
int lca(int u,int v)
{
    if(dep[u]<dep[v]) swap(u,v);
    for(int i=19;i>=0;i--)
    {
        if(dep[fa[u][i]]>=dep[v]) u=fa[u][i];
    }
    if(u==v) return v;
    for(int i=19;i>=0;i--)
    {
        if(fa[u][i]!=fa[v][i])
        {
            u=fa[u][i],v=fa[v][i];
        }
    }
    return fa[u][0];
}
void cal_sum(int u,int father)
{
    for(int i=0;i<edge[u].size();i++)
    {
        int son=edge[u][i];
        if(son==father) continue;
        cal_sum(son,u);
        w[u]+=w[son];
    }
}
signed main()
{
    cin>>n>>m;
    for(int i=1;i<=n-1;i++)
    {
        int x,y;cin>>x>>y;
        edge[x].push_back(y);
        edge[y].push_back(x);
        id[{x,y}]=i;
        id[{y,x}]=i;
    }
    dfs(1,0);
    for(int i=0;i<m;i++)
    {
        int a,b;cin>>a>>b;
        w[a]++,w[b]++;
        w[lca(a,b)]-=2;
    }
    cal_sum(1,0);
    int ans=-1;
    for(int i=1;i<=n;i++)
    {
        if(w[i]==m)
        {
            int ID=id[{i,fa[i][0]}];
            ans=max(ans,ID);
        }
    }
    cout<<ans;
    return 0;
}