Problem p02696 (C++)

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

// long long
using ll = long long;

// pair<int, int>
using PII = pair<int, int>;

// 最大値、mod
const int MOD = 1000000007;
const int mod = 1000000007;
const int INF = 1000000000;
const long long LINF = 1e18;
const int MAX = 510000;

// 出力系
#define print(x) cout << x << endl
#define prints(x) cout << fixed << setprecision(10) << x << endl
#define printc(x) cout << setw(2) << setfill('0') << x << endl;
#define yes cout << "Yes" << endl
#define YES cout << "YES" << endl
#define no cout << "No" << endl
#define NO cout << "NO" << endl

// begin() end()
#define all(x) (x).begin(), (x).end()

// for
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)

// 最大公約数
unsigned gcd(unsigned a, unsigned b) {
  if (a < b)
    return gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

// 最小公倍数
unsigned lcm(unsigned a, unsigned b) { return a / gcd(a, b) * b; }

// a = max(a, b), a = min(a, b)
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

// 階乗(MODをとる)
ll pow_mod(ll num, ll pow, ll mod) {
  ll prod = 1;
  num %= mod;
  while (pow > 0) {
    if (pow & 1)
      prod = prod * num % mod;
    num = num * num % mod;
    pow >>= 1;
  }
  return prod;
}

// 二項係数(MODとる、1 ≦ k ≦ n ≦ 10^7 程度)
// COMinit()
// COM(x, y)
// とコンビで使う

// テーブルを作る前処理
long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

// int dx[4] = {0, 0, 1, -1}, dy[4] = {1, -1, 0, 0};

struct UnionFind {
  vector<int> par;
  vector<int> rank;
  vector<ll> Size;
  UnionFind(int n = 1) { init(n); }

  void init(int n = 1) {
    par.resize(n + 1);
    rank.resize(n + 1);
    Size.resize(n + 1);
    for (int i = 0; i <= n; ++i)
      par[i] = i, rank[i] = 0, Size[i] = 1;
  }

  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    Size[x] += Size[y];
    return true;
  }

  ll size(int x) { return Size[root(x)]; }
};
#define rrep(i, a, b) for (int i = (a); i > (b); i--)
#define rep(i, a, b) for (int i = (a); i < (b); i++)

struct Mint {
  int val;
  Mint inv() const {
    int tmp, a = val, b = mod, x = 1, y = 0;
    while (b)
      tmp = a / b, a -= tmp * b, swap(a, b), x -= tmp * y, swap(x, y);
    return Mint(x);
  }

public:
  Mint() : val(0) {}
  Mint(ll x) {
    if ((val = x % mod) < 0)
      val += mod;
  }
  Mint pow(ll t) {
    Mint res = 1, b = *this;
    while (t) {
      if (t & 1)
        res *= b;
      b *= b;
      t >>= 1;
    }
    return res;
  }
  Mint &operator+=(const Mint &x) {
    if ((val += x.val) >= mod)
      val -= mod;
    return *this;
  }
  Mint &operator-=(const Mint &x) {
    if ((val += mod - x.val) >= mod)
      val -= mod;
    return *this;
  }
  Mint &operator*=(const Mint &x) {
    val = (ll)val * x.val % mod;
    return *this;
  }
  Mint &operator/=(const Mint &x) { return *this *= x.inv(); }
  bool operator==(const Mint &x) const { return val == x.val; }
  bool operator!=(const Mint &x) const { return val != x.val; }
  bool operator<(const Mint &x) const { return val < x.val; }
  bool operator<=(const Mint &x) const { return val <= x.val; }
  bool operator>(const Mint &x) const { return val > x.val; }
  bool operator>=(const Mint &x) const { return val >= x.val; }
  Mint operator+(const Mint &x) const { return Mint(*this) += x; }
  Mint operator-(const Mint &x) const { return Mint(*this) -= x; }
  Mint operator*(const Mint &x) const { return Mint(*this) *= x; }
  Mint operator/(const Mint &x) const { return Mint(*this) /= x; }
};
struct factorial {
  vector<Mint> Fact, Finv;

public:
  factorial(int maxx) {
    Fact.resize(maxx + 1), Finv.resize(maxx + 1);
    Fact[0] = Mint(1);
    rep(i, 0, maxx) Fact[i + 1] = Fact[i] * (i + 1);
    Finv[maxx] = Mint(1) / Fact[maxx];
    rrep(i, maxx, 0) Finv[i - 1] = Finv[i] * i;
  }
  Mint fact(int n, bool inv = 0) {
    if (inv)
      return Finv[n];
    else
      return Fact[n];
  }
  Mint nPr(int n, int r) {
    if (n < 0 || n < r || r < 0)
      return Mint(0);
    else
      return Fact[n] * Finv[n - r];
  }
  Mint nCr(int n, int r) {
    if (n < 0 || n < r || r < 0)
      return Mint(0);
    else
      return Fact[n] * Finv[r] * Finv[n - r];
  }
};

ll modfact(ll n) {
  if (n <= 1)
    return 1;
  return (n * modfact(n - 1)) % MOD;
}

int main() {
  ll a, b, n;
  cin >> a >> b >> n;
  if (b - 1 > n) {
    cout << (a * n) / b - a * (n / b) << endl;
    return 0;
  }
  ll x = (a * (b - 1)) / b - a * ((b - 1) / b);
  ll pos = (n - (n % (b - 1)));
  ll y = (a * pos) / b - a * (pos / b);
  x > y ? print(x) : print(y);
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;

// long long
using ll = long long;

// pair<int, int>
using PII = pair<int, int>;

// 最大値、mod
const int MOD = 1000000007;
const int mod = 1000000007;
const int INF = 1000000000;
const long long LINF = 1e18;
const int MAX = 510000;

// 出力系
#define print(x) cout << x << endl
#define prints(x) cout << fixed << setprecision(10) << x << endl
#define printc(x) cout << setw(2) << setfill('0') << x << endl;
#define yes cout << "Yes" << endl
#define YES cout << "YES" << endl
#define no cout << "No" << endl
#define NO cout << "NO" << endl

// begin() end()
#define all(x) (x).begin(), (x).end()

// for
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)

// 最大公約数
unsigned gcd(unsigned a, unsigned b) {
  if (a < b)
    return gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

// 最小公倍数
unsigned lcm(unsigned a, unsigned b) { return a / gcd(a, b) * b; }

// a = max(a, b), a = min(a, b)
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

// 階乗(MODをとる)
ll pow_mod(ll num, ll pow, ll mod) {
  ll prod = 1;
  num %= mod;
  while (pow > 0) {
    if (pow & 1)
      prod = prod * num % mod;
    num = num * num % mod;
    pow >>= 1;
  }
  return prod;
}

// 二項係数(MODとる、1 ≦ k ≦ n ≦ 10^7 程度)
// COMinit()
// COM(x, y)
// とコンビで使う

// テーブルを作る前処理
long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

// int dx[4] = {0, 0, 1, -1}, dy[4] = {1, -1, 0, 0};

struct UnionFind {
  vector<int> par;
  vector<int> rank;
  vector<ll> Size;
  UnionFind(int n = 1) { init(n); }

  void init(int n = 1) {
    par.resize(n + 1);
    rank.resize(n + 1);
    Size.resize(n + 1);
    for (int i = 0; i <= n; ++i)
      par[i] = i, rank[i] = 0, Size[i] = 1;
  }

  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    Size[x] += Size[y];
    return true;
  }

  ll size(int x) { return Size[root(x)]; }
};
#define rrep(i, a, b) for (int i = (a); i > (b); i--)
#define rep(i, a, b) for (int i = (a); i < (b); i++)

struct Mint {
  int val;
  Mint inv() const {
    int tmp, a = val, b = mod, x = 1, y = 0;
    while (b)
      tmp = a / b, a -= tmp * b, swap(a, b), x -= tmp * y, swap(x, y);
    return Mint(x);
  }

public:
  Mint() : val(0) {}
  Mint(ll x) {
    if ((val = x % mod) < 0)
      val += mod;
  }
  Mint pow(ll t) {
    Mint res = 1, b = *this;
    while (t) {
      if (t & 1)
        res *= b;
      b *= b;
      t >>= 1;
    }
    return res;
  }
  Mint &operator+=(const Mint &x) {
    if ((val += x.val) >= mod)
      val -= mod;
    return *this;
  }
  Mint &operator-=(const Mint &x) {
    if ((val += mod - x.val) >= mod)
      val -= mod;
    return *this;
  }
  Mint &operator*=(const Mint &x) {
    val = (ll)val * x.val % mod;
    return *this;
  }
  Mint &operator/=(const Mint &x) { return *this *= x.inv(); }
  bool operator==(const Mint &x) const { return val == x.val; }
  bool operator!=(const Mint &x) const { return val != x.val; }
  bool operator<(const Mint &x) const { return val < x.val; }
  bool operator<=(const Mint &x) const { return val <= x.val; }
  bool operator>(const Mint &x) const { return val > x.val; }
  bool operator>=(const Mint &x) const { return val >= x.val; }
  Mint operator+(const Mint &x) const { return Mint(*this) += x; }
  Mint operator-(const Mint &x) const { return Mint(*this) -= x; }
  Mint operator*(const Mint &x) const { return Mint(*this) *= x; }
  Mint operator/(const Mint &x) const { return Mint(*this) /= x; }
};
struct factorial {
  vector<Mint> Fact, Finv;

public:
  factorial(int maxx) {
    Fact.resize(maxx + 1), Finv.resize(maxx + 1);
    Fact[0] = Mint(1);
    rep(i, 0, maxx) Fact[i + 1] = Fact[i] * (i + 1);
    Finv[maxx] = Mint(1) / Fact[maxx];
    rrep(i, maxx, 0) Finv[i - 1] = Finv[i] * i;
  }
  Mint fact(int n, bool inv = 0) {
    if (inv)
      return Finv[n];
    else
      return Fact[n];
  }
  Mint nPr(int n, int r) {
    if (n < 0 || n < r || r < 0)
      return Mint(0);
    else
      return Fact[n] * Finv[n - r];
  }
  Mint nCr(int n, int r) {
    if (n < 0 || n < r || r < 0)
      return Mint(0);
    else
      return Fact[n] * Finv[r] * Finv[n - r];
  }
};

ll modfact(ll n) {
  if (n <= 1)
    return 1;
  return (n * modfact(n - 1)) % MOD;
}

int main() {
  ll a, b, n;
  cin >> a >> b >> n;
  if (b - 1 > n) {
    cout << (a * n) / b - a * (n / b) << endl;
    return 0;
  }
  if (b == 1) {
    cout << 0 << endl;
    return 0;
  }
  ll x = (a * (b - 1)) / b - a * ((b - 1) / b);
  ll pos = (n - (n % (b - 1)));
  ll y = (a * pos) / b - a * (pos / b);
  x > y ? print(x) : print(y);
}