#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR if(DEBUG) cerr
#define MACRO(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,NAME,...) NAME
#define pr(...) CERR<<MACRO(__VA_ARGS__,pr10,pr9,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__)<<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define pr9(a,b,c,d,e,f,g,h,i) pr1(a)<<pr8(b,c,d,e,f,g,h,i)
#define pr10(a,b,c,d,e,f,g,h,i,j) pr1(a)<<pr9(b,c,d,e,f,g,h,i,j)
#define prArr(a){CERR<<(#a)<<"={";int i=0;for(auto t:(a))CERR<<(i++?", ":"")<<t;CERR<<"}"<<endl;}
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE

void check(string s){

  set<Int> S;
  function<void(string, Int)> dfs= [&](string s, Int num){
    if(s.size() == 1){
      S.insert(num);
      assert(S.size() == 1);
    }
    for(Int i=0;i<(Int)s.size()-1;i++){
      string a = s.substr(i, 2);
      string b = to_string((Int)((a[0] - '0') + (a[1] - '0')));
      string ns = s.substr(0, i) + b + s.substr(i + 2);
      dfs(ns, num + 1);
    }
  };
  dfs(s, 0);
}

void test(){
  while(1){
    string num = to_string(rand()%100000000);
    pr(num);
    check(num);
  }
}

Int calcNum(string s){
  if(s == "") return 0;
  Int res = 0;
  while(s.size() != 1){
    Int N = s.size();
    Int a = s[N - 2] - '0';
    Int b = s[N - 1] - '0';
    s.pop_back();
    s.pop_back();
    s += to_string(a + b);
    res++;
  }
  return res;
}

Int solve_(Int M, vector<P> A){
  string num = "";
  for(Int i=0;i<M;i++){
    Int d, c;tie(d, c) = A[i];
    num += string(c, char(d + '0'));
  }
  pr(num);
  //check(num);
  return calcNum(num);
}

pair<Int, string > calc(Int c, Int d){
  if(c == 1) return {0, to_string(d)};
  Int num = c / 2;
  Int nc = c / 2;
  Int nd = d * 2;
  if(nd >= 10){
    nd = nd % 10+ nd/10;
    num += c / 2;
  }
  auto res = calc(nc, nd);
  res.first += num;
  if(c % 2) res.second += char(d + '0');
  return res;
}


Int solve(Int M, vector<P> A){
  string num = "";
  Int ans = 0;
  for(Int i=0;i<M;i++){
    Int d, c; tie(d, c) = A[i];
    auto t = calc(c, d);
    ans += t.first;
    num += t.second;
    pr(d, c);
  }
  pr(ans, num);
  ans += calcNum(num);
  return ans;
}

void test2(){
  while(1){
    Int M = rand()%20 + 1;
    vector<P> A(M);
    for(Int i=0;i<M;i++){
      Int d = rand() % 10;
      while(i == 0 && d == 0) d = rand()%10;
      Int c = rand()%20 + 2;
      A[i] = P(d, c);
    }
    Int a = solve(M, A);
    Int b = solve_(M, A);
    pr(M, A, a, b);
    assert(a == b);
  }

}


signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);
  //test();
  //test2();

  Int M;
  cin>>M;
  vector<P> A(M);
  cin>>A;

  Int ans = solve(M, A);
  cout<<ans<<endl;


  return 0;
}


Measured memory_bytes: 13184