/* -*- coding: utf-8 -*-
 *
 * 249.cc: No.249 N言っちゃダメゲーム (2) - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int TN = 1000;

/* typedef */

typedef long long ll;

/* global variables */

bool fws[TN];
ll dp[TN + 1][2];

/* subroutines */

/* main */

int main() {
  for (int i = 0; i < TN; i++) {
    ll ni, ki;
    cin >> ni >> ki;
    fws[i] = ((ni - 1) % (ki + 1) != 0);
  }

  dp[TN][0] = dp[TN][1] = 0;
  for (int i = TN - 1; i >= 0; i--) {
    ll dmin, dmax;
    if (dp[i + 1][0] <= dp[i + 1][1]) dmin = dp[i + 1][0], dmax = dp[i + 1][1];
    else dmin = dp[i + 1][1], dmax = dp[i + 1][0];

    ll dw = dmin + 1, dl = dmax;
    if (fws[i]) {
      dp[i][0] = dw, dp[i][1] = dl;
    }
    else {
      dp[i][0] = dw, dp[i][1] = dl;
    }
  }

  printf("%lld\n", dp[0][0]);
  return 0;
}