#include <iostream>
#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep_r(i, n) for (int i = n - 1; i >= 0; i--)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define all(x) (x).begin(), (x).end()
#define SZ(x) ((ll)(x).size())
#define bit(n) (1LL << (n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

#define INF bit(60)
#define pb push_back
#define mod 1000000007

template <class T>
bool chmax(T &a, const T &b)
{
  if (a < b)
  {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b)
{
  if (b < a)
  {
    a = b;
    return 1;
  }
  return 0;
}

using namespace std;
using uif = uint_fast64_t;
using ll = long long int;
using tTree = __gnu_pbds::tree<ll, __gnu_pbds::null_type, less<ll>, __gnu_pbds::rb_tree_tag, __gnu_pbds::tree_order_statistics_node_update>;

int qp(int a, ll b)
{
  int ans = 1;
  do
  {
    if (b & 1)
      ans = 1ll * ans * a % mod;
    a = 1ll * a * a % mod;
  } while (b >>= 1);
  return ans;
}
int qp(int a, ll b, int mo)
{
  int ans = 1;
  do
  {
    if (b & 1)
      ans = 1ll * ans * a % mo;
    a = 1ll * a * a % mo;
  } while (b >>= 1);
  return ans;
}

int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

#define FACSIZE 2002

ll invfac[FACSIZE];
ll fac[FACSIZE];

ll mod_pow(ll, ll);
ll mod_add(ll, ll);
ll mod_mul(ll, ll);
ll mod_div(ll, ll);
void make_fact(ll);
void make_invfact(ll);

int main(void)
{

  ll m;
  cin >> m;
  vector<ll> d(m);
  vector<ll> c(m);

  vector<ll> modu(m);
  deque<ll> deq;

  rep(i, m) cin >> d[i] >> c[i];
  ll ans = 0;
  for (ll i = 0; i < m; i++)
  {
    ll total = d[i] * c[i];
    if (d[i] == 0)
    {
      ans += c[i] - 1;
      modu[i] = 0;
      deq.push_back(0);
    }
    else if (total % 9LL == 0)
    {
      ans += total / 9LL + c[i] - 1 - 1;
      modu[i] = 9LL;
      deq.push_back(9);
    }
    else
    {
      ans += total / 9LL + c[i] - 1;
      modu[i] = total % 9LL;
      deq.push_back(total % 9LL);
    }
  }

  //cout << deq.size() << endl;

  while (deq.size() > 1)
  {
    auto q1 = deq.front();
    deq.pop_front();
    auto q2 = deq.front();
    deq.pop_front();
    auto val = q1 + q2;

    if (val >= 10)
    {
      deq.push_front(val % 10);
      deq.push_front(val / 10);
    }
    else
      deq.push_front(val);
    ans++;
  }

  cout << ans << endl;

  return 0;
}

ll mod_pow(ll x, ll r)
{
  if (r == 0)
    return 1;
  else if (r == 1)
    return x % mod;
  else if (r % 2 == 0)
  {
    ll t = mod_pow(x, r / 2) % mod;
    return t * t % mod;
  }
  else
  {
    ll t = mod_pow(x, r / 2) % mod;
    ll k = t * t % mod;
    return k * x % mod;
  }
}
ll mod_add(ll a, ll b)
{
  return (a + b) % mod;
}

ll mod_mul(ll a, ll b)
{
  return (a * b) % mod;
}

ll mod_div(ll a, ll b)
{
  return mod_mul(a, mod_pow(b, mod - 2));
}

void make_fact(ll n)
{
  fac[0] = 1;
  rep(i, n)
  {
    fac[i + 1] = mod_mul(fac[i], i + 1);
  }
}

void make_invfact(ll n)
{
  invfac[n] = mod_pow(fac[n], mod - 2);
  for (int i = n - 1; i >= 0; i--)
  {
    invfac[i] = mod_mul(invfac[i + 1], i + 1);
  }
}


Measured memory_bytes: 6528