#include <bits/stdc++.h>

using namespace std;


using int64 = long long;
const int mod = 998244353;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

  template< typename... Args >
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward< Args >(args)...);
  }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}


int main() {
  int M;
  cin >> M;
  vector< int64 > D(M), C(M);
  for(int i = 0; i < M; i++) {
    cin >> D[i] >> C[i];
  }

  int64 ret = 0;
  int64 last = -1;

  for(int i = 0; i < M; i++) {

    int64 tap = min(1LL, C[i]);
    C[i] -= tap;


    for(int j = 0; j < tap; j++) {
      if(last == -1) {
        last = D[i];
      } else {
        int64 now = D[i];
        while(now + last >= 10) {
          now += last;
          last = now % 10;
          now = now / 10;
          ++ret;
        }
        last += now;
        ++ret;
      }
    }


    auto table = make_v< int64 >(60, 10);
    fill_v(table, -1);
    auto cost = make_v< int64 >(60, 10);

    for(int x = 0; x < 10; x++) {
      int64 lastt = x, noww = D[i], addd = 0;
      while(lastt + noww >= 10) {
        noww += lastt;
        lastt = noww % 10;
        noww = noww / 10;
        addd++;
      }
      addd++;
      lastt += noww;
      table[0][x] = lastt;
      cost[0][x] = addd;
    }

    for(int k = 0; k + 1 < 60; k++) {
      for(int j = 0; j < 10; j++) {
        if(table[k][j] == -1) {
          table[k + 1][j] = -1;
          cost[k + 1][j] = cost[k][j];
        } else {
          table[k + 1][j] = table[k][table[k][j]];
          cost[k + 1][j] = cost[k][j] + cost[k][table[k][j]];
        }
      }
    }

    for(int64 j = 59; j >= 0; j--) {
      if((C[i] >> j) & 1) {
        ret += cost[j][last];
        last = table[j][last];
      }
    }


  }
  cout << ret << endl;
}






Measured memory_bytes: 3456