Problem p03821 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cmath>
#include <iostream>
#include <limits>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef int itn;

const int inf = numeric_limits<int>::max() / 4;
const ll linf = numeric_limits<ll>::max() / 4;
const ull ulinf = numeric_limits<ull>::max() / 4;
const double pi = acos(-1);
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const int dx8[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
const int dy8[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

#define p_queue(i) priority_queue<i>
#define rp_queue(i) priority_queue<i, vector<i>, greater<i>>
#define umap(i, j) unordered_map<i, j>
#define P(p) cout << (p) << endl
#define PS(p) cout << (p) << " "
#define rep(i, m, n) for (int i = (m); i < (int)(n); i++)
#define mod(i) ((i) % (ll)(1e9 + 7))
#define divm(a, b) (mod(a * modpow((ll)b, (ll)(1e9 + 5))))
#define rsort(a, b, c) sort(a, b, greater<c>())
#define vsort(v) sort(v.begin(), v.end());
#define rvsort(v) sort(v.end(), v.begin());
#define ft first
#define sd second
#define pb push_back
#define it insert
#define sz(x) ((int)(x).size())
#define lb(a, n, k) (lower_bound(a, a + n, k) - a)
#define vlb(a, k) (lower_bound(a.begin(), a.end(), k) - a.begin())
#define ub(a, n, k) (upper_bound(a, a + n, k) - a)
#define vub(a, k) (upper_bound(a.begin(), a.end(), k) - a.begin())
#define YES cout << "YES" << endl
#define NO cout << "NO" << endl
#define Yes cout << "Yes" << endl
#define No cout << "No" << endl
#define yes cout << "yes" << endl
#define no cout << "no" << endl
#define ret return

ll modpow(ll i, ll j) {
  ll tmp = 1;
  while (j) {
    if (j % 2)
      tmp = mod(tmp * i);
    i = mod(i * i);
    j /= 2;
  }
  return tmp;
}
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

vector<string> split(const string &str, char sep) {
  vector<string> v;
  stringstream ss(str);
  string buffer;
  while (getline(ss, buffer, sep)) {
    v.push_back(buffer);
  }
  return v;
}

// ll ncr[100][100];
// ll nCr(int n, int r){if(n==r) ret ncr[n][r] = 1; if(r==0) ret ncr[n][r] = 1;
// if(r==1) ret ncr[n][r] = n;if(ncr[n][r]) ret ncr[n][r]; ret ncr[n][r] =
// nCr(n-1,r) + nCr(n-1,r-1);}

// ll npr[100][100]={};
// ll nPr(int n,int r){if(npr[n][r])ret npr[n][r];if(r==0)ret npr[n][r] =
// 1;if(r==1)ret npr[n][r] = n;ret npr[n][r] = n * nPr(n-1,r-1);}

// ll nHr(int n,int r){ret nCr(n+r-1,r);}

///////////////////////////////////////////////////////////////////////////

int main() {

  ll n;
  cin >> n;
  ll a[10000] = {}, b[100005] = {};

  rep(i, 0, n) cin >> a[i] >> b[i];

  ll cnt = 0;
  rep(i, 0, n) {
    ll ri = n - i - 1;

    ll m = ((a[ri] + cnt) % b[ri]);
    if (m != 0)
      cnt += b[ri] - m;
  }
  P(cnt);
  return 0;
}

Fixed submission:
#include <algorithm>
#include <cmath>
#include <iostream>
#include <limits>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef int itn;

const int inf = numeric_limits<int>::max() / 4;
const ll linf = numeric_limits<ll>::max() / 4;
const ull ulinf = numeric_limits<ull>::max() / 4;
const double pi = acos(-1);
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const int dx8[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
const int dy8[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

#define p_queue(i) priority_queue<i>
#define rp_queue(i) priority_queue<i, vector<i>, greater<i>>
#define umap(i, j) unordered_map<i, j>
#define P(p) cout << (p) << endl
#define PS(p) cout << (p) << " "
#define rep(i, m, n) for (int i = (m); i < (int)(n); i++)
#define mod(i) ((i) % (ll)(1e9 + 7))
#define divm(a, b) (mod(a * modpow((ll)b, (ll)(1e9 + 5))))
#define rsort(a, b, c) sort(a, b, greater<c>())
#define vsort(v) sort(v.begin(), v.end());
#define rvsort(v) sort(v.end(), v.begin());
#define ft first
#define sd second
#define pb push_back
#define it insert
#define sz(x) ((int)(x).size())
#define lb(a, n, k) (lower_bound(a, a + n, k) - a)
#define vlb(a, k) (lower_bound(a.begin(), a.end(), k) - a.begin())
#define ub(a, n, k) (upper_bound(a, a + n, k) - a)
#define vub(a, k) (upper_bound(a.begin(), a.end(), k) - a.begin())
#define YES cout << "YES" << endl
#define NO cout << "NO" << endl
#define Yes cout << "Yes" << endl
#define No cout << "No" << endl
#define yes cout << "yes" << endl
#define no cout << "no" << endl
#define ret return

ll modpow(ll i, ll j) {
  ll tmp = 1;
  while (j) {
    if (j % 2)
      tmp = mod(tmp * i);
    i = mod(i * i);
    j /= 2;
  }
  return tmp;
}
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

vector<string> split(const string &str, char sep) {
  vector<string> v;
  stringstream ss(str);
  string buffer;
  while (getline(ss, buffer, sep)) {
    v.push_back(buffer);
  }
  return v;
}

// ll ncr[100][100];
// ll nCr(int n, int r){if(n==r) ret ncr[n][r] = 1; if(r==0) ret ncr[n][r] = 1;
// if(r==1) ret ncr[n][r] = n;if(ncr[n][r]) ret ncr[n][r]; ret ncr[n][r] =
// nCr(n-1,r) + nCr(n-1,r-1);}

// ll npr[100][100]={};
// ll nPr(int n,int r){if(npr[n][r])ret npr[n][r];if(r==0)ret npr[n][r] =
// 1;if(r==1)ret npr[n][r] = n;ret npr[n][r] = n * nPr(n-1,r-1);}

// ll nHr(int n,int r){ret nCr(n+r-1,r);}

///////////////////////////////////////////////////////////////////////////

int main() {

  ll n;
  cin >> n;
  ll a[100005] = {}, b[100005] = {};

  rep(i, 0, n) cin >> a[i] >> b[i];

  ll cnt = 0;
  rep(i, 0, n) {
    ll ri = n - i - 1;

    ll m = ((a[ri] + cnt) % b[ri]);
    if (m != 0)
      cnt += b[ri] - m;
  }
  P(cnt);
  return 0;
}