#include <bits/stdc++.h>
#define x first
#define y second
#define int long long
using namespace std;
typedef pair<int,int> PII;
const int N = 1e6+10,mod = 998244353,INF=1e12,cap  = 20210926;
int W,ans;
int dfs()
{
    int x,y;
    cin>>x>>y;
    vector<int>nums(y+1,0);
    for(int i=1;i<=y;i++)   nums[i]=dfs();
    int res=0;
    for(int i=1;i<=y;i++)
    {
        if(res+nums[i]>W)
        {
            ans++;
            res=0;
        }
        res+=nums[i];
    }
    res+=x;
    ans+=res/W;
    return res%W;
}
void solve()
{
    cin>>W;
    int res = dfs();
    if(res) ans++;
    cout<<ans<<"\n";
}

signed main()
{
    int T=1;
    cin.tie(0)->sync_with_stdio(false);
    //cin>>T;
    //init();
    while(T--)
    {
       solve();
    }

    return 0;
}
/*
5
2 3
2 2
1 0
1 0
6 0
4 2
3 0
2 0

7
5 2
7 0
2 0
 */