// lanqiao3517 砍树(LCA+树上差分)
#include<bits/stdc++.h>
using namespace std;
const int N = 5e5 + 10;
// 链式前向量存储图
int edges[N], next_edge[N], head[N], edge_idx;
int fa[N][25], depth[N], subtree_pairs[N];
int n, m;
// 添加一条边
void addEdge(int s, int t){
    edges[edge_idx] = t;
    next_edge[edge_idx] = head[s];
    head[s] = edge_idx++;
}
// 深度搜索优先并记录每个节点的深度与祖先信息
void dfs(int cur, int parent, int dep){
    fa[cur][0] = parent;
    depth[cur] = dep;
    for(int i = 1; i <= 20; i++){
        fa[cur][i] = fa[fa[cur][i-1]][i-1]; // 动态规划处理快速求最近
    }
    for(int i = head[cur]; i != -1; i = next_edge[i]){
        int child = edges[i];
        if(child == parent) continue;
        dfs(child, cur, dep + 1);
    }
}
int LCA(int x, int y){
    if(depth[x] < depth[y]) swap(x, y);
    // 将x向上跳到与y同一深度或更浅的位置
    for(int i = 20; i >= 0; i--){
        if(depth[fa[x][i]] >= depth[y]){
            x = fa[x][i];
        }
    }
    if(x == y) return x;
    for(int i = 20; i >= 0; i--){
        if(fa[x][i] != fa[y][i]){
            x = fa[x][i];
            y = fa[y][i];
        }
    }
    return fa[x][0]; // 返回最近公共祖先
}
void updateSubtreePairs(int cur, int parent, int edgeId, int& maxEdgeId){
    for(int i = head[cur]; i != -1; i = next_edge[i]){
        int child = edges[i];
        if(child == parent) continue;
        updateSubtreePairs(child, cur, i, maxEdgeId); // 递归更新子节点
        subtree_pairs[cur] += subtree_pairs[child];
    }
    // 当前节点及其子树内包含m对点对时，更新答案
    if(subtree_pairs[cur] == m){
        maxEdgeId = max(maxEdgeId, edgeId / 2 + 1); // 边的编号减半(因为是无向图, 一条边有两个方向的编号)
    }
}
int main(){
    cin >> n >> m;
    // 初始化邻接表
    for(int i = 1; i <= n; i++) head[i] = -1;
    // 读入树的边并建立邻接表
    for(int i = 1; i < n; i++){
        int u, v;
        cin >> u >> v;
        addEdge(u, v); addEdge(v, u);
    }
    // 深度优先搜索预处理节点深度与祖先信息
    dfs(1, 0, 1);
    for(int i = 1; i <= m; i++){
        int firstNode, secondNode;
        cin >> firstNode >> secondNode;
        subtree_pairs[firstNode]++; 
        subtree_pairs[secondNode]++;
        subtree_pairs[LCA(firstNode, secondNode)] -= 2;
    }
    int ans = -1;
    updateSubtreePairs(1, 0, 0, ans);
    cout << ans;
    return 0;
}
/* test samples -> 4
6 2
1 2
2 3
4 3
2 5
6 5
3 6
4 5
*/