#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <cfloat>
#include <ctime>
#include <cassert>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <numeric>
#include <list>
#include <iomanip>
#include <fstream>
   
using namespace std;
   

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define foreach(it, c) for (__typeof__((c).begin()) it=(c).begin(); it != (c).end(); ++it)
#define rforeach(it, c) for (__typeof__((c).rbegin()) it=(c).rbegin(); it != (c).rend(); ++it)
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define CL(arr, val) memset(arr, val, sizeof(arr))
#define COPY(dest, src) memcpy(dest, src, sizeof(dest))
#define ten(n) ((long long)(1e##n))
#define bin(n) (1LL << (n))

template <class T> void max_swap(T& a, const T& b) { a = max(a, b); }
template <class T> void min_swap(T& a, const T& b) { a = min(a, b); }

template <class T> void uniq(vector<T>& c) { sort(c.begin(), c.end()); c.erase(unique(c.begin(), c.end()), c.end()); }

template <class T> string to_s(const T& a) { ostringstream os; os << a; return os.str(); }
template <class T> T to_T(const string& s) { istringstream is(s); T res; is >> res; return res; }
  
template <class T, class U> ostream& operator<<(ostream& os, pair<T, U>& p) { os << "( " << p.first << ", " << p.second << " )"; return os; }
  
template <class T> void print(T a, int n, const string& deli = " ", int br = 1) { for (int i = 0; i < n; ++i) { cout << a[i]; if (i + 1 != n) cout << deli; } while (br--) cout << endl; }
template <class T> void print(const T& c, const string& deli = " ", int br = 1) { foreach (it, c) { cout << *it; if (++it != c.end()) cout << deli;--it; } while (br--) cout << endl; }
template <class T> void print2d(T a, int w, int h, int width = -1, int br = 1) { for (int i = 0; i < h; ++i) { for (int j = 0; j < w; ++j) {    if (width != -1) cout.width(width); cout << a[i][j] << ' '; } cout << endl; } while (br--) cout << endl; }
   
template <class T> void input(T& a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }
template <class T> void input(T* a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }

void fast_io() { cin.tie(0); ios::sync_with_stdio(false); }
#define trace(x) (cout << #x << ": " << (x) << endl)

bool valid(int x, int y, int w, int h) { return 0 <= x && x < w && 0 <= y && y < h; }


typedef long long ll;
typedef pair<int, int> pint;

const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { 1, 0, -1, 0 };
  
const double PI = acos(-1.0);
const int mod = 1000000007;



int main()
{
    int n;
    ll max_h;
    ll d[ten(5) + 100], h[ten(5) + 100];
    scanf("%d%lld", &n, &max_h);
    rep(i, n - 1)
        scanf("%lld%lld", d + i, h + i);

    ll sum_d[ten(5) + 100];
    sum_d[0] = 0;
    rep(i, n - 1)
        sum_d[i + 1] = sum_d[i] + d[i];

    ll res = 0, hp = max_h;
    deque<int> q;
    rep(i, n - 1)
    {
        while (!q.empty() && sum_d[i] - sum_d[q.front()] >= max_h)
            q.pop_front();
        while (!q.empty() && h[q.back()] <= h[i])
            q.pop_back();
        q.push_back(i);

        while (hp <= d[i])
        {
            int k = q.front();
#define heal_upper() (max_h - (hp + (sum_d[i] - sum_d[k])))

            ll full_heal_use_upper = max(0LL, heal_upper() / h[k]);
            ll need_hp = d[i] - hp + 1;
            ll use = min(full_heal_use_upper, (need_hp + h[k] - 1) / h[k]);
            res += use;
            hp += use * h[k];

            if (hp <= d[i])
            {
                ll hh = heal_upper();
                if (q.size() >= 2 && h[q[1]] >= hh)
                    q.pop_front();
                else
                {
                    ++res;
                    hp += hh;
                }
            }
        }

        hp -= d[i];
    }

    printf("%lld\n", res);
}