#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define vi vector<int>
#define pii pair<int,int>
#define pll pair<ll,ll>
#define vll vector<ll>
#define FOR(i,n) for(int i=0;i<n;i++)
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define pb push_back
#define f first
#define s second

const int MOD = (int)1e9+7;
ll w;
ll ans = 0;
ll create(){
    ll height,branches; 
    cin>>height>>branches;
    ll remain = height;
    vll br;
    FOR(i,branches){
        ll x = create();
        if(x)
        br.pb(x);
        remain+=x;
    }
    sort(br.begin(),br.end());
    while(sz(br)>1 && remain>w){
        //cout<<"SEG"<<br.back()<<endl;
        if(br.back())
        ans++; 
        remain-=br.back();
        br.pop_back();
    }
    ll k = remain/w;
    ans+=k;
    remain-=k*w;
    return remain;
}

int main()
{
    cin>>w;
    ll x = create();
    ans+=ceil(x/(double)w);
    cout<<ans<<endl;
    return 0;
}