#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <iomanip>
using namespace std;

const int N = 2e5 + 5, inf = 1e9;
#define pb push_back
#define mp make_pair
#define ll long long
#define ff first
#define ss second
#define all(a) a.begin(),a.end()
#define sz(x) (int)x.size()
typedef vector<int> vi;
typedef pair<int,int> ii;
typedef vector<ii> vii;

vii g[N];
int cur = 2;
void scan(int n, int par){
    for(int i = 0; i < n; i++){
        int a, b; cin >> a >> b;
        cur++;
        g[par].pb({cur, a});
        scan(b, cur);
    }
}

ll dp[N];

void dfs(int u){
    for(auto [v, w] : g[u]){
        dfs(v);
        dp[u] += (dp[v] + w);
    }
}
void solve(){
    ll w, m;
    cin >> w >> m;
    int n;
    cin >> n;
    g[1].pb({2, m});
    
    scan(n, 2);
    dfs(1);

    cout << (dp[1] + w - 1) / w << '\n';
}

int main(){
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    int t = 1;
    // cin >> t;
    while(t--){
        solve();
    }
    return 0;
}