#ifdef LOCAL111
	#define _GLIBCXX_DEBUG
#else
	#pragma GCC optimize ("O3")
	#define NDEBUG
#endif
// #define _USE_MATH_DEFINES
#include <bits/stdc++.h>
const int INF = 1e9;
using namespace std;
template<typename T, typename U> ostream& operator<< (ostream& os, const pair<T,U>& p) { os << '(' << p.first << ' ' << p.second << ')'; return os; }

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) int((a).size())
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n)  FOR(i,0,n)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#ifdef LOCAL111
	#define DEBUG(x) cout<<#x<<": "<<(x)<<endl
	template<typename T> void dpite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
#else
	#define DEBUG(x) true
	template<typename T> void dpite(T a, T b){ return; }
#endif
#define F first
#define S second
#define SNP string::npos
#define WRC(hoge) cout << "Case #" << (hoge)+1 << ": "
template<typename T> void pite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
template<typename T> bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;}
template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}

template<typename T>
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 U,typename... V>
typename enable_if<is_same<T, U>::value!=0>::type
fill_v(U &u,const V... v){u=U(v...);}

template<typename T,typename U,typename... V>
typename enable_if<is_same<T, U>::value==0>::type
fill_v(U &u,const V... v){
  for(auto &e:u) fill_v<T>(e,v...);
}

const array<int, 4> dx = {0, 1, 0, -1};
const array<int, 4> dy = {1, 0, -1, 0};


typedef long long int LL;
typedef unsigned long long ULL;
typedef pair<int,int> P;

void ios_init(){
	//cout.setf(ios::fixed);
	//cout.precision(12);
#ifdef LOCAL111
	return;
#endif
	ios::sync_with_stdio(false); cin.tie(0);
}

pair<int, LL> countv(vector<int> v) {
	if(SZ(v) == 1) return {v[0], 0};
	int x = v[0];
	LL res = 0;
	FOR(i, 1, SZ(v)) {
		x += v[i];
		res++;
		if(x > 9) {
			res++;
			x = x / 10 + x % 10;
		}
	}
	return {x, res};
}

pair<int, LL> count(int d, LL c) {
	if(c == 1) return {d, 0};
	LL res = 0;
	LL resd = 0;
	if(d + d > 9) {
		res += c / 2 * 2;
		DEBUG(c / 2 + c / 2 / 2);
		LL nd = (d + d) / 10 + (d + d) % 10;
		LL r;
		int rd;
		tie(rd, r) = count(nd, c / 2);
		res += r;
		vector<int> v;
		v.push_back(rd);
		if(c % 2) {
			v.push_back(d);
		}
		
		DEBUG(1);
		dpite(ALL(v));
		LL t;
		tie(resd, t) = countv(v);
		res += t;
	} else {
		res += c / 2;	
		LL nd = d + d;
		LL r;
		int rd;
		tie(rd, r) = count(nd, c / 2);
		// DEBUG(rd);
		res += r;
		vector<int> v;
		v.push_back(rd);
		if(c % 2) {
			v.push_back(d);
		}
		LL t;
		tie(resd, t) = countv(v);
		res += t;
	}
	return {resd, res};
}

int main()
{
	ios_init();
	int m;
	while(cin >> m) {
		vector<int> d(m);
		vector<LL> c(m);
		REP(i, m) cin >> d[i] >> c[i];
		LL ans = 0;
		vector<int> v;
		REP(i, m) {
			LL t;
			int dv;
			tie(dv, t) = count(d[i], c[i]);
			DEBUG(dv); DEBUG(t);
			v.push_back(dv);
			ans += t;
		}
		dpite(ALL(v));
		LL x;
		tie(ignore, x) = countv(v);
		ans += x;
		cout << ans << endl;
	}

	return 0;
}


Measured memory_bytes: 5240