// Why I am so dumb? :c
#include <bits/stdc++.h>

#define pb push_back
#define mp make_pair

#define all(x) (x).begin(), (x).end()

#define fi first
#define se second

#define pii pair<int, int>

using namespace std;

typedef long long ll;

const int MAXN = (int)2e3 + 5;

vector<int> adj[MAXN];

int tmp[MAXN];

int n, m;

void dfs(int v, int pr = -1) {
    for (int to : adj[v]) {
        if (to == pr) {
            continue;
        }

        tmp[to] = v;
        dfs(to, v);
    }
}

vector<pii> calc(vector<pii> e, vector<pii> arr) {
    int v = -1, par = -1;
    map<int, int> deg;

    for (pii it : e) {
        ++deg[it.fi];
        ++deg[it.se];
    }

    for (pii it : deg) {
        if (it.se == 1) {
            v = it.fi;
        }
    }

    if (v == -1) {
        return arr;
    }

    for (pii it : e) {
        if (it.fi == v || it.se == v) {
            par = it.fi + it.se - v;
        }
    }

    vector<pii> newArr, newE;
    vector<int> leaf;
    map<int, int> p;

    for (int i = 0; i < arr.size(); ++i) {
        if (arr[i].fi == arr[i].se) {
            continue;
        }

        if (arr[i].fi == v) {
            swap(arr[i].fi, arr[i].se);
        }

        if (arr[i].se == v) {
            leaf.pb(i);            
        }
        else {
            p[newArr.size()] = i;
            newArr.pb(arr[i]);
        }
    }

    vector<pair<pii, int>> dep;

    while (leaf.size() >= 2) {
        int a = leaf.back();
        leaf.pop_back();
        int b = leaf.back();
        leaf.pop_back();
        
        dep.pb(mp(mp(a, b), newArr.size()));
        newArr.pb(mp(arr[a].fi, arr[b].fi));
    }

    if (leaf.size() == 1) {
        int a = leaf.back();
        leaf.pop_back();

        dep.pb(mp(mp(a, a), newArr.size()));
        newArr.pb(mp(arr[a].fi, par));
    }

    for (pii it : e) {
        if (it.fi != v && it.se != v) {
            newE.pb(it);
        }
    }

    vector<pii> sub = calc(newE, newArr);
    vector<pii> cur = arr;

    for (pii it : p) {
        cur[it.se] = sub[it.fi];
    }

    for (auto it : dep) {
        int a = it.fi.fi, b = it.fi.se;

        if (a == b) {
            if (sub[it.se] != newArr[it.se]) {
                swap(cur[a].fi, cur[a].se);   
            }

            continue;
        }

        if (sub[it.se] == newArr[it.se]) {
            swap(cur[b].fi, cur[b].se);
        }
        else {
            swap(cur[a].fi, cur[a].se);
        }
    }

    return cur;
}

void solve() {                   
    scanf("%d %d", &n, &m);
    vector<pii> e(n - 1), arr(m);
    map<pii, int> cnt;

    for (int i = 0; i < n - 1; ++i) {
        scanf("%d %d", &e[i].fi, &e[i].se);
        adj[e[i].fi].pb(e[i].se);
        adj[e[i].se].pb(e[i].fi);
    }

    for (int i = 0; i < m; ++i) {
        scanf("%d %d", &arr[i].fi, &arr[i].se);
        dfs(arr[i].fi);

        for (int v = arr[i].se; v != arr[i].fi; v = tmp[v]) {
            if (tmp[v] < v) {
                ++cnt[mp(tmp[v], v)];
            }
            else {
                ++cnt[mp(v, tmp[v])];
            }
        }
    }

    int ans = 0;

    for (auto it : cnt) {
        ans += min(2, it.se);
    }

    printf("%d\n", ans);
    vector<pii> vv = calc(e, arr);

    for (pii it : vv) {
        printf("%d %d\n", it.fi, it.se);
    }
}

int main() {    
    int tt = 1;

    while (tt--) {
        solve();
    }

    return 0;
}