#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;
using pl = pair<ll, ll>;
using vl = vector<ll>;

#define f first
#define s second

ll k;
ll ans = 0;

ll f() {
    ll x, n;
    cin >> x >> n;
    while (n--) {
        x += f();
    }
    ll t = x / k;
    ans += t;
    x %= k;
    return x;
}

int main() {
    cin >> k;
    ll r = f();
    ans += (r + k - 1) / k;
    cout << ans << '\n';
}