Problem p03821 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <functional>
#include <iostream>
#include <limits.h>
#include <map>
#include <math.h>
#include <queue>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <unordered_map>
#include <vector>
using namespace std;

#define intmax INT_MAX
#define lmax LONG_MAX
#define uintmax UINT_MAX
#define ulmax ULONG_MAX
#define llmax LLONG_MAX
#define ll long long
#define rep(i, a, N) for ((i) = (a); (i) < (N); (i)++)
#define rrp(i, N, a) for ((i) = (N)-1; (i) >= (a); (i)--)
#define llfor ll i, j, k
#define sc(a) cin >> a
#define pr(a) cout << a << endl
#define pY puts("YES")
#define pN puts("NO")
#define py puts("Yes")
#define pn puts("No")
#define pnn printf("\n")
#define sort(a) sort(a.begin(), a.end())
#define push(a, b) (a).push_back(b)
#define llvec vector<vector<ll>>
#define charvec vector<vector<char>>
#define sizeoof(a, b) (a, vector<ll>(b))
#define llpvec vector<pair<ll, ll>>
/*繰り上げ除算*/ ll cei(ll x, ll y) {
  ll ans = x / y;
  if (x % y != 0)
    ans++;
  return ans;
}
/*最大公約数*/ ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
/*最小公倍数*/ ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }
/*n乗*/ ll llpow(ll x, ll n) {
  ll i, ans = 1;
  rep(i, 0, n) ans *= x;
  return ans;
}
/*階乗*/ ll fact(ll x) {
  ll i, ans = 1;
  rep(i, 0, x) ans *= (x - i);
  return ans;
}
/*nCr*/ ll ncr(ll n, ll r) { return fact(n) / fact(r) / fact(n - r); }
/*nPr*/ ll npr(ll n, ll r) { return fact(n) / fact(n - r); }
/*primejudge*/ bool prime(ll a) {
  if (a <= 1)
    return false;
  ll i;
  for (i = 2; i * i <= a; i++) {
    if (a % i == 0)
      return false;
  }
  return true;
}
ll ans = 0;
llfor; ///////////////////////////////////////////////////////////
int main() {
  ll n;
  sc(n);
  ll a[n + 10], b[n + 10];
  rep(i, 0, n) {
    sc(a[i]);
    sc(b[i]);
  }
  ans = 0;
  rrp(i, n, 0) {
    a[i] += ans;
    while (a[i] % b[i] != 0) {
      ans++;
      a[i]++;
      //  cout<<i<<" "<<ans<<" "<<a[i]<<" "<<b[i]<<endl;
    }
  }
  pr(ans);

  return 0;
}

Fixed submission:
#include <algorithm>
#include <functional>
#include <iostream>
#include <limits.h>
#include <map>
#include <math.h>
#include <queue>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <unordered_map>
#include <vector>
using namespace std;

#define intmax INT_MAX
#define lmax LONG_MAX
#define uintmax UINT_MAX
#define ulmax ULONG_MAX
#define llmax LLONG_MAX
#define ll long long
#define rep(i, a, N) for ((i) = (a); (i) < (N); (i)++)
#define rrp(i, N, a) for ((i) = (N)-1; (i) >= (a); (i)--)
#define llfor ll i, j, k
#define sc(a) cin >> a
#define pr(a) cout << a << endl
#define pY puts("YES")
#define pN puts("NO")
#define py puts("Yes")
#define pn puts("No")
#define pnn printf("\n")
#define sort(a) sort(a.begin(), a.end())
#define push(a, b) (a).push_back(b)
#define llvec vector<vector<ll>>
#define charvec vector<vector<char>>
#define sizeoof(a, b) (a, vector<ll>(b))
#define llpvec vector<pair<ll, ll>>
/*繰り上げ除算*/ ll cei(ll x, ll y) {
  ll ans = x / y;
  if (x % y != 0)
    ans++;
  return ans;
}
/*最大公約数*/ ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
/*最小公倍数*/ ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }
/*n乗*/ ll llpow(ll x, ll n) {
  ll i, ans = 1;
  rep(i, 0, n) ans *= x;
  return ans;
}
/*階乗*/ ll fact(ll x) {
  ll i, ans = 1;
  rep(i, 0, x) ans *= (x - i);
  return ans;
}
/*nCr*/ ll ncr(ll n, ll r) { return fact(n) / fact(r) / fact(n - r); }
/*nPr*/ ll npr(ll n, ll r) { return fact(n) / fact(n - r); }
/*primejudge*/ bool prime(ll a) {
  if (a <= 1)
    return false;
  ll i;
  for (i = 2; i * i <= a; i++) {
    if (a % i == 0)
      return false;
  }
  return true;
}
ll ans = 0;
llfor; ///////////////////////////////////////////////////////////
int main() {
  ll n;
  sc(n);
  ll a[n + 10], b[n + 10];
  rep(i, 0, n) {
    sc(a[i]);
    sc(b[i]);
  }
  ans = 0;
  rrp(i, n, 0) {
    if ((a[i] + ans) % b[i] == 0)
      continue;
    ans += b[i] - (a[i] + ans) % b[i];
    // cout<<i<<" "<<ans<<endl;
  }
  pr(ans);

  return 0;
}