// May this submission get accepted!

#pragma GCC optimize ("O3")
#pragma GCC target ("tune=native")
#pragma GCC target ("avx")

#include <bits/stdc++.h>

// 汎用マクロ
#define ALL_OF(x) (x).begin(), (x).end()
#define REP(i,n) for (long long i=0, i##_len=(n); i<i##_len; i++)
#define RANGE(i,is,ie) for (long long i=(is), i##_end=(ie); i<=i##_end; i++)
#define DSRNG(i,is,ie) for (long long i=(is), i##_end=(ie); i>=i##_end; i--)
#define STEP(i, is, ie, step) for (long long i=(is), i##_end=(ie), i##_step = (step); i<=i##_end; i+=i##_step)
#define UNIQUE(v) do { sort((v).begin(), (v).end()); (v).erase(unique((v).begin(), (v).end()), (v).end()); } while (false)
template<class T> bool chmax(T &a, const T &b) { if (a < b) {a = b; return true;} return false; }
template<class T> bool chmin(T &a, const T &b) { if (a > b) {a = b; return true;} return false; }
#define INF 0x7FFFFFFF
#define LINF 0x7FFFFFFFFFFFFFFFLL
#define Yes(q) ((q) ? "Yes" : "No")
#define YES(q) ((q) ? "YES" : "NO")
#define Possible(q) ((q) ? "Possible" : "Impossible")
#define POSSIBLE(q) ((q) ? "POSSIBLE" : "IMPOSSIBLE")
#define DUMP(q) cerr << "[DEBUG] " #q ": " << (q) << " at " __FILE__ ":" << __LINE__ << endl
#define DUMPALL(q) do { cerr << "[DEBUG] " #q ": ["; REP(DUMPALL_ITR, (q).size()) { cerr << (q)[DUMPALL_ITR] << (DUMPALL_ITR == DUMPALL_ITR_len-1 ? "" : ", "); } cerr << "] at " __FILE__ ":" << __LINE__ << endl; } while (false)
template<class T> T gcd(T a, T b) { if (a < b) std::swap(a, b); while (b) std::swap(a %= b, b); return a; }
template<class T> T lcm(const T a, const T b) { return a / gcd(a, b) * b; }

// gcc拡張マクロ
#define popcount __builtin_popcount
#define popcountll __builtin_popcountll

// エイリアス
#define DANCE_ long
#define ROBOT_ unsigned
#define HUMAN_ signed
using  ll = DANCE_ HUMAN_ DANCE_;
using ull = DANCE_ ROBOT_ DANCE_;
using  ld = long double;
using namespace std;

// モジュール

void debug_output(vector<ll> &d, vector<ll> &c) {
    string s;
    REP(i, d.size()) {
        s += string(c[i], d[i] + '0');
    }
    DUMP(s);
}

// 処理内容
int main() {
    
    // インタラクティブ問題では除去した方がいいかも
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    
    ll m; cin >> m;
    vector<ll> d, c;
    d.reserve(m*2);
    c.reserve(m*2);
    REP(i, m) cin >> d[i] >> c[i];

    ll ans = 0;
    REP(i, m) {
        if (c[i] == 1) continue;
        ll drem = -1;
        while (c[i] > 1) {
            bool is_odd = c[i] % 2 != 0;
            ll dold = d[i];
            c[i] /= 2;
            ans += c[i];
            d[i] *= 2;
            if (d[i] > 9) {
                ans += c[i];
                d[i] -= 9;
            }
            if (is_odd) {
                if (drem == -1) {
                    drem = dold;
                } else {
                    drem += dold;
                    ans++;
                    if (drem > 9) {
                        ans++;
                        drem -= 9;
                    }
                }
            }
        }
        if (drem != -1) {
            d[i] += drem;
            ans++;
            if (d[i] > 9) {
                d[i] -= 9;
                ans++;
            }
        }
    }
    DSRNG(i, m-2, 0) {
        d[i] += d[i+1];
        ans++;
        if (d[i] > 9) {
            d[i] -= 9;
            ans++;
        }
    }

    cout << ans << endl;
    
}

Measured memory_bytes: 4480