Problem p03103 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;

void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V> void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T> void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x)
    cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V> void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v))
    cerr << ", ";
  _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)                                                            \
  cerr << "[" << #x << "] = [";                                                \
  _print(x)
#else
#define debug(x...)
#endif

#define lp(i, a, b) for (int i = a; i < (b); i++)
#define lpd(i, a, b) for (int i = (b)-1; i >= a; i--)

#define sz(x) (int)(x).size()
#define all(x) x.begin(), x.end()
#define mp make_pair
#define pb push_back

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef vector<int> vi;
typedef vector<ll> vl;

bool cmp(pl a, pl b) { return (a.first <= b.first); }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  int n;
  ll m, a, b;
  cin >> n >> m;

  vector<pair<ll, ll>> v(n);
  lp(i, 0, n) {
    cin >> a >> b;
    v[i] = mp(a, b);
  }
  sort(all(v), cmp);

  ll sum = 0, price = 0;
  int i = 0;
  while (sum + v[i].second <= m) {
    sum += v[i].second;
    price += (v[i].first * v[i].second);
    i++;
  }
  price += (v[i].first * (m - sum));

  cout << price << endl;

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>

using namespace std;

void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V> void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T> void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x)
    cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V> void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v))
    cerr << ", ";
  _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)                                                            \
  cerr << "[" << #x << "] = [";                                                \
  _print(x)
#else
#define debug(x...)
#endif

#define lp(i, a, b) for (int i = a; i < (b); i++)
#define lpd(i, a, b) for (int i = (b)-1; i >= a; i--)

#define sz(x) (int)(x).size()
#define all(x) x.begin(), x.end()
#define mp make_pair
#define pb push_back

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef vector<int> vi;
typedef vector<ll> vl;

bool cmp(pl a, pl b) { return (a.first < b.first); }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  int n;
  ll m, a, b;
  cin >> n >> m;

  vector<pair<ll, ll>> v(n);
  lp(i, 0, n) {
    cin >> a >> b;
    v[i] = mp(a, b);
  }
  sort(all(v), cmp);

  ll sum = 0, price = 0;
  int i = 0;
  while (sum + v[i].second <= m) {
    sum += v[i].second;
    price += (v[i].first * v[i].second);
    i++;
  }
  price += (v[i].first * (m - sum));

  cout << price << endl;

  return 0;
}