//MLEが動かない
//セグフォは関数内で要素のでかい配列を定義しているから
//todo　idef DEBUGで、境界チェック付きのvectorを作りたい
//http://marycore.jp/prog/cpp/class-extension-methods/
//現状ifdef DEBUG が何故か動かないので出来ない

#include <bits/stdc++.h>

using namespace std;
#define int long long
#define ll long long
#define double long double

struct initon {
    initon() {
        cin.tie(0);
        ios::sync_with_stdio(false);
        cout.setf(ios::fixed);
        cout.precision(16);
    };
} hoee;

namespace std {
    template<>
    class hash<std::pair<signed, signed>> {
    public:
        size_t operator()(const std::pair<signed, signed> &x) const {
            return hash<ll>()(((ll) x.first << 32) + x.second);
        }
    };

    template<>
    class hash<std::pair<ll, ll>> {
    public:
        size_t operator()(const std::pair<ll, ll> &x) const {
            return hash<ll>()(((ll) x.first << 32) + x.second);
        }
    };
}
struct Tri {
    int f, s, t;

    Tri() {
        f = -1, s = -1, t = -1;
    }

    Tri(int f, int s, int t) : f(f), s(s), t(t) {}

    bool operator<(const Tri &r) const {
        return f != r.f ? f < r.f : s != r.s ? s < r.s : t < r.t;
        //return f != r.f ? f > r.f : s != r.s ? s > r.s : t > r.t; 大きい順
    }

    bool operator>(const Tri &r) const {
        return f != r.f ? f > r.f : s != r.s ? s > r.s : t > r.t;
        //return f != r.f ? f > r.f : s != r.s ? s > r.s : t > r.t; 小さい順
    }

    int operator[](int i) {
        assert(i < 3);
        switch (i) {
            case 0:
                return f;
                break;
            case 1:
                return s;
                break;
            default:
                return t;
                break;
        }
    }
};
//別名
#define ull unsigned long long
#define vec vector
#define con continue
#define bre break
#define brk break
#define is ==
#define eq ==
#define no !=
#define ne !=
#define dif !=
#define df !=
#define rs resize
#define PQ priority_queue<ll, vector<ll>, greater<ll> >
using seti = set<int>;
using vi = vector<int>;
#define vvi(a, b, c) vec<vi> a(b,vi(c))
using vb = vector<bool>;
#define vvb(a, b, c) vec<vb> a(b,vb(c))
using vs = vector<string>;
#define vvs(a, b, c) vec<vs> a(b,vs(c))
using vl = vector<ll>;
#define vvl(a, b, c) vec<vl> a(b,vl(c))
using vd = vector<double>;
#define vvd(a, b, c) vec<vd> a(b,vd(c))
#define v3i(a, b, c, d) vector<vector<vi>> a(b, vector<vi>(c, vi(d)))
#define v3d(a, b, c, d) vector<vector<vd>> a(b, vector<vd>(c, vd(d)))
#define v3m(a, b, c, d) vector<vector<vm>> a(b, vector<vm>(c, vm(d)))
using vc=vector<char>;
#define vvc(a, b, c) vec<vc> a(b,vc(c))
using P = pair<int, int>;
//using T = tuple<int, int, int>;
using vp = vector<P>;
#define vvp(a, b, c) vec<vp> a(b,vp(c))
using vt = vector<Tri>;
#define vvt(a, b, c) vec<vt> a(b,vt(c))
using dou = double;
using itn = int;
using str = string;
using bo= bool;
#define uset unordered_set
#define mset multiset
#define umap unordered_map
#define mmap multimap
#define F first
#define S second

//定数
#define k4 10101
#define k5 101010
#define k6 1010101
#define k7 10101010

const int INF = (int) 1e9 + 100;
const ll LINF = (ll) 1e18 + 100;
const double EPS = 1e-9;
const int y4[] = {-1, 1, 0, 0};
const int x4[] = {0, 0, -1, 1};
const int y8[] = {0, 1, 0, -1, -1, 1, 1, -1};
const int x8[] = {1, 0, -1, 0, 1, -1, 1, -1};

//配列
#define sz(a) (sizeof(a)/sizeof(a[0]))

//コンテナ
#define mp make_pair
#define pb push_back
#define pf push_front
#define eb emplace_back
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()

template<class T>
T pop(set<T> &set) {
    T res = *set.begin();
    set.erase(set.find(res));
    return res;
}

template<class T>
T pop(mset<T> &set) {
    T res = *set.begin();
    set.erase(set.find(res));
    return res;
}

template<class T>
T popBack(set<T> &set) {
    T res = *set.rbegin();
    set.erase(set.find(res));
    return res;
}

template<class T>
T popBack(mset<T> &set) {
    T res = *set.rbegin();
    set.erase(set.find(res));
    return res;
}

template<class T>
inline void sort(vector<T> &a) { sort(a.begin(), a.end()); };

template<class T>
inline void rsort(vector<T> &a) { sort(a.begin(), a.end(), greater<T>()); };

template<class T>
inline void sort(vector<T> &a, int len) { sort(a.begin(), a.begin() + len); };

template<class T>
inline void rsort(vector<T> &a, int len) { sort(a.begin(), a.begin() + len, greater<T>()); };


//繰り返し
#define _overloadrep(_1, _2, _3, name, ...) name
# define _rep(i, n) for(int i = 0; i < n ; i++)
#define repi(i, m, n) for(int i = m; i < n ; i++)
#define rep(...) _overloadrep(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#define _rer(i, n) for(int i = n; i >= 0 ; i--)
#define reri(i, m, n) for(int i = m; i >= n ; i--)
#define rer(...) _overloadrep(__VA_ARGS__,reri,_rer,)(__VA_ARGS__)
#define fora(a, b) for(auto&& a : b)
#define forr(a, b) for_each(map.rbegin(),map.rend(),[](auto&& a)
#define rea(a, b) for(auto&& a : b)
#define repa(a, b) for(auto&& a : b)

template<typename A, size_t N, typename T>
void fill(A (&a)[N], const T &v) {
    rep(i, N)a[i] = v;
}

template<typename A, size_t N, size_t O, typename T>
void fill(A (&a)[N][O], const T &v) {
    rep(i, N)rep(j, O)a[i][j] = v;
}

template<typename A, size_t N, size_t O, size_t P, typename T>
void fill(A (&a)[N][O][P], const T &v) {
    rep(i, N)rep(j, O)rep(k, P)a[i][j][k] = v;
}

template<typename A, size_t N, size_t O, size_t P, size_t Q, typename T>
void fill(A (&a)[N][O][P][Q], const T &v) {
    rep(i, N)rep(j, O)rep(k, P)rep(l, Q)a[i][j][k][l] = v;
}

template<typename A, size_t N, size_t O, size_t P, size_t Q, size_t R, typename T>
void fill(A (&a)[N][O][P][Q][R], const T &v) {
    rep(i, N)rep(j, O)rep(k, P)rep(l, Q)rep(m, R)a[i][j][k][l][m] = v;
}

template<typename A, size_t N, size_t O, size_t P, size_t Q, size_t R, size_t S, typename T>
void fill(A (&a)[N][O][P][Q][R][S], const T &v) {
    rep(i, N)rep(j, O)rep(k, P)rep(l, Q)rep(m, R)rep(n, S)a[i][j][k][l][m][n] = v;
}

template<class T, class U>
void fill(vector<T> &a, U v) {
    rep(i, a.size())a[i] = v;
}

template<class T, class U>
void fill(vector<vector<T>> &a, U v) {
    rep(i, a.size())rep(j, a[0].size())a[i][j] = v;
}

template<class T, class U>
void fill(vector<vector<vector<T>>> &a, U v) {
    rep(i, a.size())rep(j, a[0].size())rep(k, a[0][0].size())a[i][j][k] = v;
}

#define arcpy(a, b) memcpy(a,b,sizeof(b))

//入力
template<typename T = int>
T in() {
    T x;
    cin >> x;
    return (x);
}

string sin() { return in<string>(); }

double din() { return in<double>(); }

ll lin() { return in<ll>(); }

#define na(a, n) rep(i,n) cin >> a[i];
#define nad(a, n) rep(i,n) cin >> a[i], a[i]--;
#define na3(a, b, c, n) rep(i, n)cin >> a[i] >> b[i] >> c[i];
#define add2(a, b, n) rep(i, n)a.pb(in()),b.pb(in());
#define add2d(a, b, n) rep(i, n)a.pb(in()-1),b.pb(in()-1);
#define add3(a, b, c, n) rep(i, n)a.pb(in()),b.pb(in()),c.pb(in());
#define add3d(a, b, c, n) rep(i, n)a.pb(in()-1),b.pb(in()-1),c.pb(in());
#define na2(a, b, n) rep(i, n)cin >> a[i] >> b[i];

#define nt(a, h, w) rep(hi,h)rep(wi,w) cin >> a[hi][wi];
#define ntd(a, h, w) rep(hi,h)rep(wi,w) cin >> a[hi][wi], a[hi][wi]--;
#define ntp(a, h, w) fill(a,'#');rep(hi,1,h+1)rep(wi,1,w+1) cin >> a[hi][wi];

#define addn(a, n) a.resize(n);na(a,n);
#define addnd(a, n) a.resize(n);na(a,n);rep(i,n)a[i]--;


//出力
template<class T>
void out(vector<T> x) {
    for (auto &&v : x) cout << v << " ";
    cout << "" << endl;
}

template<class T>
void out(T x) {
    if (typeid(x) == typeid(double))cout << fixed << setprecision(10) << x << endl;
    else cout << x << endl;
}
//デバッグ
#define debug(x) cerr << x << " " << "(L:" << __LINE__ << ")" << '\n';
// 境界チェック付きvector
namespace std_vector_bounds_checking {
    using namespace std;

    template<class T, class A = std::allocator<T>>
    struct vector : std::vector<T, A> {
        using std::vector<T, A>::vector;

        typename std::vector<T>::reference operator[](typename std::vector<T>::size_type n) {
            return this->at(n);
        }
    };
}
//よく使うクラス、構造体
class UnionFind {
public:
    vi par, rank, sizes;
    int n, trees;

    UnionFind(int n) : n(n), trees(n) {
        par.resize(n), rank.resize(n), sizes.resize(n);
        rep(i, n)par[i] = i, sizes[i] = 1;
    }

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

    void unite(int x, int y) {
        x = root(x);
        y = root(y);
        if (x == y)return;
        if (rank[x] < rank[y])swap(x, y);
        trees--;
        par[y] = x;
        sizes[x] += sizes[y];
        if (rank[x] == rank[y])rank[x]++;
    }

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

    int size(int x) { return sizes[root(x)]; }
    //順不同 umapなので
    vec<vi> sets() {
        vec<vi> res(trees);
        umap<int, vi> map;
        rep(i, n) map[root(i)].push_back(i);
        int i = 0;
        for (auto &&p:map) {
            int r = p.F;
            res[i].push_back(r);
            for (auto &&v:p.S) {
                if (r == v)continue;
                res[i].push_back(v);
            }
            i++;
        }
        return res;
    }
};

//MOD関連
ll MOD = (int) 1e9 + 7;

int mpow(int v, ll a) {
    ll x = v, n = a, res = 1;
    while (n) {
        if (n & 1)res = (res * x) % MOD;
        x = (x * x) % MOD;
        n >>= 1;
    }
    return res;
}

class mint {
private:
    ll v;
public:
    static ll mod(ll a) { return (a % MOD + MOD) % MOD; }

    mint(ll a = 0) { this->v = mod(a); };

    mint(const mint &a) { v = a.v; }

    mint operator+(const mint &a) { return mint(v + a.v); }

    mint operator+(const ll a) { return mint(v + a % MOD); }

    mint operator+(const signed a) { return mint(v + a % MOD); }

    friend mint operator+(const ll a, const mint &b) { return mint(a % MOD + b.v); }

    void operator+=(const mint &a) { v = (v + a.v) % MOD; }

    void operator+=(const ll a) { v = mod(v + a % MOD); }

    void operator+=(const signed a) { v = mod(v + a % MOD); }

    friend void operator+=(ll &a, const mint &b) { a = mod(a % MOD + b.v); }

    mint operator-(const mint &a) { return mint(v - a.v); }

    mint operator-(const ll a) { return mint(v - a % MOD); }

    mint operator-(const signed a) { return mint(v - a % MOD); }

    friend mint operator-(const ll a, const mint &b) { return mint(a % MOD - b.v); }

    void operator-=(const mint &a) { v = mod(v - a.v); }

    void operator-=(const ll a) { v = mod(v - a % MOD); }

    void operator-=(const signed a) { v = mod(v - a % MOD); }

    friend void operator-=(ll &a, const mint &b) { a = mod(a % MOD - b.v); }

    mint operator*(const mint &a) { return mint(v * a.v); }

    mint operator*(const ll a) { return mint(v * (a % MOD)); }

    mint operator*(const signed a) { return mint(v * (a % MOD)); }

    friend mint operator*(const ll a, const mint &b) { return mint(a % MOD * b.v); }

    void operator*=(const mint &a) { v = (v * a.v) % MOD; }

    void operator*=(const ll a) { v = mod(v * (a % MOD)); }

    void operator*=(const signed a) { v = mod(v * (a % MOD)); }

    friend void operator*=(ll &a, const mint &b) { a = mod(a % MOD * b.v); }

    mint operator/(const mint &a);

    mint operator/(const ll a);

    mint operator/(const signed a);

    friend mint operator/(const ll a, const mint &b);

    void operator/=(const mint &a);

    void operator/=(const ll a);

    void operator/=(const signed a);

    friend void operator/=(ll &a, const mint &b);

    mint operator^(const mint &a) { return mpow(v, a.v); };

    mint operator^(const ll a) { return mpow(v, a); };

    mint operator^(const signed a) { return mpow(v, a); };

    friend mint operator^(const ll a, const mint &b) { return mpow(a, b.v); };

    void operator^=(const mint &a) { v = mpow(v, a.v); }

    void operator^=(const ll a) { v = mpow(v, a); }

    void operator^=(const signed a) { v = mpow(v, a); }

    //単項演算子
    mint operator+() { return *this; }

    mint operator++() {
        v++;
        return *this;
    }

    mint operator++(signed d) {
        mint res = *this;
        v++;
        return res;
    }

    mint operator-() { return operator*(-1); }

    mint operator--() {
        v++;
        return *this;
    }

    void operator--(signed d) {
        mint res = *this;
        v++;
    }

    bool operator==(mint &a) {
        return v == a.v;
    }

    bool operator==(signed a) {
        return v == a;
    }

    friend bool operator==(signed a, mint &b) {
        return a == b.v;
    }

    bool operator!=(mint &a) {
        return v != a.v;
    }

    bool operator!=(signed a) {
        return v != a;
    }

    friend bool operator!=(signed a, mint &b) {
        return a != b.v;
    }

    operator int() { return v; }
};

const int setModMax = 510000;
mint fac[setModMax], finv[setModMax], inv[setModMax];

void setMod() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < setModMax; 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;
    }
}

mint minv(ll a) {
    if (fac[0] == 0)setMod();
    if (a < setModMax) return inv[a];
    a %= MOD;
    ll b = MOD, x = 1, y = 0;
    while (b) {
        ll t = a / b;
        a -= t * b;
        swap(a, b);
        x -= t * y;
        swap(x, y);
    }
    return (x % MOD + MOD) % MOD;
}

mint mint::operator/(const mint &a) { return mint(v * minv(a.v)); }

mint mint::operator/(const ll a) { return mint(v * minv(a)); }

mint mint::operator/(const signed a) { return mint(v * minv(a)); }

mint operator/(const ll a, const mint &b) { return mint(a % MOD * minv(b.v)); }

void mint::operator/=(const mint &a) { v = (v * minv(a.v)) % MOD; }

void mint::operator/=(const ll a) { v = mod(v * minv(a)); }

void mint::operator/=(const signed a) { v = mod(v * minv(a)); }

void operator/=(ll &a, const mint &b) { a = mint::mod(a % MOD * minv(b.v)); }

using vm=vector<mint>;
#define vvm(a, b, c) vec<vm> a(b,vm(c))
bool isPrime[4010101];
vi primes;

void setPrime() {
    fill(isPrime, true);
    isPrime[0] = isPrime[1] = false;
    for (int i = 2; i <= sqrt(sz(isPrime)) + 5; ++i) {
        if (!isPrime[i])continue;
        for (int j = 2; i * j < sz(isPrime); ++j) {
            isPrime[i * j] = false;
        }
    }
    rep(i, sz(isPrime))if (isPrime[i])primes.pb(i);
}

mint com(ll n, ll r) {
    if (n < r || n < 0 || r < 0)return 0;
    if (fac[0] == 0)setMod();
    return fac[n] * (finv[r] * finv[n - r] % MOD) % MOD;
}

//便利関数
void OLE() {
    string a = "a";
    rep(i, 30)a += a;
    rep(i, 1 << 17)cout << a << endl;
    cout << "OLE 出力長制限超過" << endl;
    exit(0);
}

void TLE() {
    int a = 1;
    while (a)cout << a << endl;
}

void RE() {
    int a = 1 / (x4[0] / MOD);
    cout << a << endl;
}

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;
}

bool equal(double a, double b) {
    return fabs(a - b) < EPS;
}

ll reverse(ll a) {
    ll res = 0;
    while (a) {
        res *= 10;
        res += a % 10;
        a /= 10;
    }
    return res;
}

ll ceil(ll a, ll b) {
    if (b == 0) {
        OLE();
        return -1;
    } else return (a + b - 1) / b;
}

ll sqrt(ll a) {
    if (a < 0)OLE();
    ll res = (ll) std::sqrt(a);
    while (res * res < a)res++;
    return res;
}

double log(double e, double x) {
    return log(x) / log(e);
}

ll sig(ll t) { return (1 + t) * t / 2; }

ll sig(ll s, ll t) { return (s + t) * (t - s + 1) / 2; }

vi divisors(int v) {
    vi res;
    for (int i = 1; i <= sqrt(v); ++i) {
        if (v % i == 0) {
            res.pb(i);
            if (i != v / i)res.pb(v / i);
        }
    }
    return res;
}

vi factorization(int v) {
    int tv = v;
    vi res;
    if (!isPrime[2])setPrime();
    for (auto &&p :primes) {
        if (v % p == 0)res.push_back(p);
        while (v % p == 0) {
            v /= p;
        }
        if (v == 1 || p * p > tv)break;
    }
    if (v > 1)res.pb(v);
    return res;
}

unordered_map<int, int> factorizationMap(int v) {
    int tv = v;
    unordered_map<int, int> res;
    if (!isPrime[2])setPrime();
    for (auto &&p :primes) {
        while (v % p == 0) {
            res[p]++;
            v /= p;
        }
        if (v == 1 || p * p > tv)break;
    }
    if (v > 1)res[v]++;
    return res;
}

int get(int a, int keta) {
    a /= (int) pow(10, keta);
    return a % 10;
}

int keta(int v) {
    int cou = 0;
    while (v) {
        cou++;
        v %= 10;
    }
    return cou;
}

template<class T>
void imo(vector<T> &v) {
    int n = v.size();
    rep(i, n - 1)v[i + 1] += v[i];
}

//変換系
template<class T, class U>
vector<U> keys(map<T, U> a) {
    vector<U> res;
    for (auto &&k :a)res.pb(k.F);
    return res;
}

template<class T, class U>
vector<U> keys(umap<T, U> a) {
    vector<U> res;
    for (auto &&k :a)res.pb(k.F);
    return res;
}

template<class T, class U>
vector<T> values(map<T, U> a) {
    vector<T> res;
    for (auto &&k :a)res.pb(k.S);
    return res;
}

template<class T, class U>
vector<T> values(umap<T, U> a) {
    vector<T> res;
    for (auto &&k :a)res.pb(k.S);
    return res;
}

vi list(int a) {
    vi res;
    while (a) {
        res.insert(res.begin(), a % 10);
        a /= 10;
    }
    return res;
}

template<class T, class U>
bool chmax(T &a, const U &b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T, class U>
bool chmin(T &a, const U &b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
T min(T a, signed b) {
    return a < b ? a : b;
}

template<class T>
T max(T a, signed b) {
    return a < b ? b : a;
}

template<class T>
T min(vector<T> a) {
    return *min_element(all(a));
}

template<class T>
T max(vector<T> a) {
    return *max_element(all(a));
}

template<class T>
T min(T a[]) {
    T res = a[0];
    rep(i, sz(a))chmin(res, a[i]);
    return res;
}

template<class T>
T max(T a[]) {
    T res = a[0];
    rep(i, sz(a))chmax(res, a[i]);
    return res;
}

template<class T>
T sum(vector<T> &v, int len = -1) {
    if (len == -1)len = v.size();
    T res = 0;
    rep(i, min((int) len, (int) v.size()))res += v[i];
    return res;
}

///要素が0の時、返り値は0か1か
template<class T>
T mul(vector<T> &v, int len = -1) {
    if (len == -1)len = v.size();
    T res = 1;
    rep(i, min((int) len, (int) v.size()))res *= v[i];
    return res;
}

void clear(PQ &q) {
    while (q.size())q.pop();
}

template<class T>
void clear(queue<T> &q) {
    while (q.size())q.pop();
}

template<class T>
vector<T> ruiv(vector<T> &a) {
    vector<T> res(a.size() + 1);
    rep(i, a.size())res[i + 1] = res[i] + a[i];
    return res;
}

template<class T>
void plus(vector<T> &a, T v = 1) {
    for (auto &&u :a)u += v;
}

template<class T>
void minu(vector<T> &a, T v = 1) {
    for (auto &&u :a)u -= v;
}

template<class T>
void minus(vector<T> &a, T v = 1) {
    for (auto &&u :a)u -= v;
}

inline bool inside(int y, int x, int H, int W) { return y >= 0 && x >= 0 && y < H && x < W; }

ll u(ll a) { return a < 0 ? 0 : a; }

#define MIN(a) numeric_limits<a>::min()
#define MAX(a) numeric_limits<a>::max()

template<class T>
T min(vector<vector<T>> &a) {
    T res = MAX(T);
    rep(i, a.size())chmin(res, *min_element(all(a[i])));
    return res;
}

template<class T>
T max(vector<vector<T>> &a) {
    T res = MIN(T);
    rep(i, a.size())chmax(res, *max_element(all(a[i])));
    return res;
}

bool bget(ll m, int keta) {
    return (m >> keta) & 1;
}

int bget(ll m, int keta, int sinsuu) {
    m /= (ll) pow(sinsuu, keta);
    return m % sinsuu;
}

inline ll bit(int n) {
    return (1LL << (n));
}

inline ll bit(int n, int sinsuu) {
    return (ll) pow(sinsuu, n);
}

int bcou(ll m) {
    return __builtin_popcount(m & 0xFFFFFFFF) + __builtin_popcount(m >> 32);
}
//初期化は0を渡す
ll nextComb(ll &mask, int n, int r) {
    if (!mask)return mask = (1LL << r) - 1;
    ll x = mask & -mask; //最下位の1
    ll y = mask + x; //連続した下の1を繰り上がらせる
    ll res = ((mask & ~y) / x >> 1) | y;
    if (bget(res, n))return mask = 0;
    else return mask = res;
}

//n桁以下でビットがr個立っているもののvectorを返す
vl bitCombList(int n, int r) {
    vl res;
    int m = 0;
    while (nextComb(m, n, r)) {
        res.pb(m);
    }
    return res;
}

int altoi(char c) {
    if ('A' <= c && c <= 'Z')return c - 'A';
    return c - 'a' + 26;
}

char itoal(int i) {
    if (i < 26)return 'A' + i;
    return 'a' + i - 26;
}

int ctoi(char c) {
    return c - '0';
}

#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

void compress(vi &a) {
    vi b;
    int len = a.size();
    for (int i = 0; i < len; ++i) {
        b.push_back(a[i]);
    }
    sort(b);
    UNIQUE(b);
    for (int i = 0; i < len; ++i) {
        a[i] = lower_bound(all(b), a[i]) - b.begin();
    }
}

void compress(int a[], int len) {
    vi b;
    for (int i = 0; i < len; ++i) {
        b.push_back(a[i]);
    }
    sort(b);
    UNIQUE(b);
    for (int i = 0; i < len; ++i) {
        a[i] = lower_bound(all(b), a[i]) - b.begin();
    }
}
//要素が見つからなかったときに困る
#define binarySearch(a, v) (binary_search(all(a),v))
#define lowerIndex(a, v) (lower_bound(all(a),v)-a.begin())
#define lowerBound(a, v) (*lower_bound(all(a),v))
#define upperIndex(a, v) (upper_bound(all(a),v)-a.begin())
#define upperBound(a, v) (*upper_bound(all(a),v))
#define ans(a) cout<<a<<endl;continue;
#define poll(a) q.front();q.pop()
#define dpoll(a) q.front();q.pop_front()
#define pollLast(a) q.back();q.pop_back()
#define pollBack(a) q.back();q.pop_back()

template<class T>
inline void fin(T s) { cout << s << endl, exit(0); }

template<class T>
struct edge {
    int from, to;
    T cost;
    int id;
    int type;

    edge(int f, int t, T c = 1, int id = -1, int ty = -1) : from(f), to(t), cost(c), id(id), type(ty) {}

    bool operator<(const edge &b) const {
        return cost < b.cost;
    }

    bool operator>(const edge &b) const {
        return cost > b.cost;
    }
};

template<typename T>
class graph {
protected:
    vector<bool> _used;
public :
    vector<vector<edge<T>>> g;
    vector<edge<T>> edges;
    int n, root = -1;

    graph(int n) : n(n) {
        g.resize(n);
        _used.resize(n);
    }

    void clear() {
        g.clear();
        edges.clear();
    }

    void resize(int n) {
        this->n = n;
        g.resize(n);
        _used.resize(n);
    }

    int size() {
        return g.size();
    }

    bool isleaf(int v) {
        assert(root != -1);
        return g[v].size() == 1 && g[v][0].from != root;
    }

    vector<edge<T> > operator[](int i) {
        return g[i];
    }

    virtual void add(int from, int to, T cost, int ty) = 0;

    virtual bool used(edge<T> &e) = 0;

    virtual bool used(int id) = 0;

    virtual void del(edge<T> &e) = 0;

    virtual void del(int id) = 0;
};

template<class T=int>
class undigraph : public graph<T> {
public:
    using graph<T>::g;
    using graph<T>::n;
    using graph<T>::edges;
    using graph<T>::_used;

    undigraph(int n) : graph<T>(n) {
    }

    void add(int f, int t, T cost = 1, int ty = -1) {
        if (!(0 <= f && f < n && 0 <= t && t < n))OLE();
        int id = edges.size();
        g[f].emplace_back(f, t, cost, id, ty);
        g[t].emplace_back(t, f, cost, id + 1, ty);
        edges.emplace_back(f, t, cost, id, ty);
        edges.emplace_back(t, f, cost, id + 1, ty);
    }

    void add(edge<T> &e) {
        int f = e.from, t = e.to, ty = e.type;
        T cost = e.cost;
        add(f, t, cost, ty);
    }

    bool used(edge<T> &e) {
        return _used[e.id];
    }

    bool used(int id) {
        return _used[id];
    }

    void del(edge<T> &e) {
        _used[e.id] = _used[e.id ^ 1] = 1;
    }

    void del(int id) {
        _used[id] = _used[id ^ 1] = 1;
    }
};

template<typename T =ll>
class digraph : public graph<T> {
public:
    using graph<T>::g;
    using graph<T>::n;
    using graph<T>::edges;
    using graph<T>::_used;

    digraph(int n) : graph<T>(n) {
    }

    void add(int f, int t, T cost = 1, int ty = -1) {
        if (!(0 <= f && f < n && 0 <= t && t < n))OLE();
        int id = edges.size();
        g[f].emplace_back(f, t, cost, ty, id);
        edges.emplace_back(f, t, cost, ty, id);
    }

    bool used(edge<T> &e) {
        return _used[e.id];
    }

    bool used(int id) {
        return _used[id];
    }

    void del(edge<T> &e) {
        _used[e.id] = _used[e.id ^ 1] = 1;
    }

    void del(int id) {
        _used[id] = _used[id ^ 1] = 1;
    }
};

template<class T>
bool nibu(const graph<T> &g) {
    UnionFind uf(g.n * 2);
    for (auto &&e :g.edges)uf.unite(e.f, e.t + g.n), uf.unite(e.f + g.n, e.t);
    return !uf.same(0, g.n);
}

template<class T>
vector<T> dijkstra(const graph<T> &g, int s) {
    if (!(0 <= s && s < g.n))OLE();
    T initValue = MAX(T);
    vector<T> dis(g.n, initValue);
    priority_queue<pair<T, int>, vector<pair<T, int>>, greater<pair<T, int>>> q;
    dis[s] = 0;
    q.emplace(0, s);
    while (q.size()) {
        T nowc = q.top().F;
        int i = q.top().S;
        q.pop();
        if (dis[i] != nowc)continue;
        for (auto &&e  : g.g[i]) {
            int to = e.to;
            T cost = nowc + e.cost;
            if (dis[to] > cost) {
                dis[to] = cost;
                q.emplace(dis[to], to);
            }
        }
    }
    //たどり着かないなら-1
    for (auto &&d :dis) if (d == initValue)d = -1;
    return dis;
}

//機能拡張
template<typename T>
void remove(vector<T> &v, unsigned int i) { v.erase(v.begin() + i); }

template<typename T>
void remove(vector<T> &v, unsigned int s, unsigned int e) {
    v.erase(v.begin() + s, v.begin() + e);
}

template<typename T>
void removen(vector<T> &v, unsigned int s, unsigned int n) {
    v.erase(v.begin() + s, v.begin() + s + n);
}

template<typename T>
void erase(vector<T> &v, unsigned int i) { v.erase(v.begin() + i); }

template<typename T>
void erase(vector<T> &v, unsigned int s, unsigned int e) {
    v.erase(v.begin() + s, v.begin() + e);
}

template<typename T>
void erasen(vector<T> &v, unsigned int s, unsigned int n) {
    v.erase(v.begin() + s, v.begin() + s + n);
}

template<typename T>
void insert(vector<T> &v, unsigned int i, T t) { v.insert(v.begin() + i, t); }

template<typename T>
void insert(vector<T> &v, unsigned int i, vector<T> list) {
    for (auto &&va :list)v.insert(v.begin() + i++, va);
}

template<typename T>
void insert(vector<T> &v, unsigned int i, initializer_list<T> list) {
    for (auto &&va :list)v.insert(v.begin() + i++, va);
}

template<typename T>
void insert(set<T> &v, vector<T> list) {
    for (auto &&va :list)v.insert(va);
}

template<typename T>
void insert(set<T> &v, initializer_list<T> list) {
    for (auto &&va :list)v.insert(va);
}

int mod(int a, int m) {
    return (a % m + m) % m;
}

ll ma = numeric_limits<ll>::min();
ll mi = numeric_limits<ll>::max();

//閉路がなければtrue
bool topo(vi &res, digraph<int> g) {
    int n = g.g.size();
    vi nyu(n);
    rep(i, n)for (auto &&e :g[i])nyu[e.to]++;
    queue<int> st;
    rep(i, n)if (nyu[i] == 0)st.push(i);
    while (st.size()) {
        int v = st.front();
        st.pop();
        res.pb(v);
        fora(e, g[v]) if (--nyu[e.to] == 0)st.push(e.to);
    }
    return res.size() == n;
}

//辞書順最小　トポロジカルソート
bool topos(vi &res, digraph<int> g) {
    int n = g.g.size();
    vi nyu(n);
    rep(i, n)for (auto &&e :g[i])nyu[e.to]++;
    set<int> st;
    rep(i, n)if (nyu[i] == 0)st.insert(i);
    while (st.size()) {
        int i = *st.begin();
        st.erase(st.begin());
        res.pb(i);
        fora(e, g[i])if (--nyu[e.to] == 0)st.insert(e.to);
    }
    return res.size() == n;
}

vector<string> split(const string a, const char deli) {
    string b = a + deli;
    int l = 0, r = 0, n = b.size();
    vector<string> res;
    rep(i, n) {
        if (b[i] == deli) {
            r = i;
            if (l < r)res.push_back(b.substr(l, r - l));
            l = i + 1;
        }
    }
    return res;
}

vector<string> split(const string a, const string deli) {
    string b = a + deli;
    int l = 0, r = 0, n = b.size(), dn = deli.size();
    vector<string> res;
    rep(i, n) {
        if (i + dn <= n && b.substr(i, i + dn) == deli) {
            r = i;
            if (l < r)res.push_back(b.substr(l, r - l));
            i += dn - 1;
            l = i + 1;
        }
    }
    return res;
}


int n, k, m, h, w, x, y, q, r;
vi a, b, c;

signed main() {
    cin >> n>> m;
    cout << n*m/2 << endl;    

    return 0;
}


Measured memory_bytes: 14080