Problem:
Score : 100 points

Problem Statement

There is a simple directed graph G with N vertices, numbered 1, 2, \ldots, N.

For each i and j (1 \leq i, j \leq N), you are given an integer a_{i, j} that represents whether there is a directed edge from Vertex i to j.
If a_{i, j} = 1, there is a directed edge from Vertex i to j; if a_{i, j} = 0, there is not.

Find the number of different directed paths of length K in G, modulo 10^9 + 7.
We will also count a path that traverses the same edge multiple times.

Constraints

All values in input are integers.

1 \leq N \leq 50

1 \leq K \leq 10^{18}

a_{i, j} is 0 or 1.

a_{i, i} = 0

Input

Input is given from Standard Input in the following format:

N K
a_{1, 1} \ldots a_{1, N}
:
a_{N, 1} \ldots a_{N, N}

Output

Print the number of different directed paths of length K in G, modulo 10^9 + 7.

Sample Input 1

4 2
0 1 0 0
0 0 1 1
0 0 0 1
1 0 0 0

Sample Output 1

6

G is drawn in the figure below:

There are six directed paths of length 2:

1 → 2 → 3

1 → 2 → 4

2 → 3 → 4

2 → 4 → 1

3 → 4 → 1

4 → 1 → 2

Sample Input 2

3 3
0 1 0
1 0 1
0 0 0

Sample Output 2

3

G is drawn in the figure below:

There are three directed paths of length 3:

1 → 2 → 1 → 2

2 → 1 → 2 → 1

2 → 1 → 2 → 3

Sample Input 3

6 2
0 0 0 0 0 0
0 0 1 0 0 0
0 0 0 0 0 0
0 0 0 0 1 0
0 0 0 0 0 1
0 0 0 0 0 0

Sample Output 3

1

G is drawn in the figure below:

There is one directed path of length 2:

4 → 5 → 6

Sample Input 4

1 1
0

Sample Output 4

0

Sample Input 5

10 1000000000000000000
0 0 1 1 0 0 0 1 1 0
0 0 0 0 0 1 1 1 0 0
0 1 0 0 0 1 0 1 0 1
1 1 1 0 1 1 0 1 1 0
0 1 1 1 0 1 0 1 1 1
0 0 0 1 0 0 1 0 1 0
0 0 0 1 1 0 0 1 0 1
1 0 0 0 1 0 1 0 0 0
0 0 0 0 0 1 0 0 0 0
1 0 1 1 1 0 1 1 1 0

Sample Output 5

957538352

Be sure to print the count modulo 10^9 + 7.

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
 
#include <bits/stdc++.h>
 
using namespace std;
 
typedef double db;
typedef long long ll;
typedef long double ld;
typedef string str;
 
typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;
typedef complex<ld> cd;
 
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<ld> vd;
typedef vector<str> vs;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;
 
#define FOR(i,a,b) for (int i = (a); i < (b); i++)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); i--)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a : x)
 
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound
 
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define rsz resize
#define ins insert
 
const int MOD = 1e9+7; // 998244353 = (119<<23)+1
const ll INF = 1e18;
const int MX = 2e5+5;
const ld PI = 4*atan((ld)1);
 
template<class T> bool ckmin(T& a, const T& b) { return a > b ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }
 
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
 
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/rope>
 
using namespace __gnu_pbds;
using namespace __gnu_cxx;
 
template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
 
#define ook order_of_key
#define fbo find_by_order
 
namespace input {
    template<class T> void re(complex<T>& x);
    template<class T1, class T2> void re(pair<T1,T2>& p);
    template<class T> void re(vector<T>& a);
    template<class T, size_t SZ> void re(array<T,SZ>& a);
 
    template<class T> void re(T& x) { cin >> x; }
    void re(double& x) { string t; re(t); x = stod(t); }
    void re(ld& x) { string t; re(t); x = stold(t); }
    template<class T, class... Ts> void re(T& t, Ts&... ts) { 
        re(t); re(ts...); 
    }
 
    template<class T> void re(complex<T>& x) { T a,b; re(a,b); x = cd(a,b); }
    template<class T1, class T2> void re(pair<T1,T2>& p) { re(p.f,p.s); }
    template<class T> void re(vector<T>& a) { F0R(i,sz(a)) re(a[i]); }
    template<class T, size_t SZ> void re(array<T,SZ>& a) { F0R(i,SZ) re(a[i]); }
}
 
using namespace input;
 
namespace output {
    void pr(int x) { cout << x; }
    void pr(long x) { cout << x; }
    void pr(ll x) { cout << x; }
    void pr(unsigned x) { cout << x; }
    void pr(unsigned long x) { cout << x; }
    void pr(unsigned long long x) { cout << x; }
    void pr(float x) { cout << x; }
    void pr(double x) { cout << x; }
    void pr(ld x) { cout << x; }
    void pr(char x) { cout << x; }
    void pr(const char* x) { cout << x; }
    void pr(const string& x) { cout << x; }
    void pr(bool x) { pr(x ? "true" : "false"); }
    template<class T> void pr(const complex<T>& x) { cout << x; }
    
    template<class T1, class T2> void pr(const pair<T1,T2>& x);
    template<class T> void pr(const T& x);
    
    template<class T, class... Ts> void pr(const T& t, const Ts&... ts) { 
        pr(t); pr(ts...); 
    }
    template<class T1, class T2> void pr(const pair<T1,T2>& x) { 
        pr("{",x.f,", ",x.s,"}"); 
    }
    template<class T> void pr(const T& x) { 
        pr("{"); // const iterator needed for vector<bool>
        bool fst = 1; for (const auto& a: x) pr(!fst?", ":"",a), fst = 0; 
        pr("}");
    }
    
    void ps() { pr("\n"); } // print w/ spaces
    template<class T, class... Ts> void ps(const T& t, const Ts&... ts) { 
        pr(t); if (sizeof...(ts)) pr(" "); ps(ts...); 
    }
    
    void pc() { pr("]\n"); } // debug w/ commas
    template<class T, class... Ts> void pc(const T& t, const Ts&... ts) { 
        pr(t); if (sizeof...(ts)) pr(", "); pc(ts...); 
    }
    #define dbg(x...) pr("[",#x,"] = ["), pc(x);
}
 
using namespace output;
 
namespace io {
    void setIn(string s) { freopen(s.c_str(),"r",stdin); }
    void setOut(string s) { freopen(s.c_str(),"w",stdout); }
    void setIO(string s = "") {
        cin.sync_with_stdio(0); cin.tie(0); // fast I/O
        cin.exceptions(cin.failbit); // ex. throws exception when you try to read letter into int
        if (sz(s)) { setIn(s+".in"), setOut(s+".out"); } // for USACO
    }
}
 
using namespace io;
 
template<class T> T invGeneral(T a, T b) {
	a %= b; if (a == 0) return b == 1 ? 0 : -1;
	T x = invGeneral(b,a); 
	return x == -1 ? -1 : ((1-(ll)b*x)/a+b)%b;
}
 
template<class T> struct modular {
	T val; 
	explicit operator T() const { return val; }
	modular() { val = 0; }
	modular(const ll& v) { 
		val = (-MOD <= v && v <= MOD) ? v : v % MOD;
		if (val < 0) val += MOD;
	}
	
	// friend ostream& operator<<(ostream& os, const modular& a) { return os << a.val; }
	friend void pr(const modular& a) { pr(a.val); }
	friend void re(modular& a) { ll x; re(x); a = modular(x); }
   
	friend bool operator==(const modular& a, const modular& b) { return a.val == b.val; }
	friend bool operator!=(const modular& a, const modular& b) { return !(a == b); }
	friend bool operator<(const modular& a, const modular& b) { return a.val < b.val; }
 
	modular operator-() const { return modular(-val); }
	modular& operator+=(const modular& m) { if ((val += m.val) >= MOD) val -= MOD; return *this; }
	modular& operator-=(const modular& m) { if ((val -= m.val) < 0) val += MOD; return *this; }
	modular& operator*=(const modular& m) { val = (ll)val*m.val%MOD; return *this; }
	friend modular pow(modular a, ll p) {
		modular ans = 1; for (; p; p /= 2, a *= a) if (p&1) ans *= a;
		return ans;
	}
	friend modular inv(const modular& a) { 
		auto i = invGeneral(a.val,MOD); assert(i != -1);
		return i;
	} // equivalent to return exp(b,MOD-2) if MOD is prime
	modular& operator/=(const modular& m) { return (*this) *= inv(m); }
	
	friend modular operator+(modular a, const modular& b) { return a += b; }
	friend modular operator-(modular a, const modular& b) { return a -= b; }
	friend modular operator*(modular a, const modular& b) { return a *= b; }
	
	friend modular operator/(modular a, const modular& b) { return a /= b; }
};
 
typedef modular<int> mi;
typedef pair<mi,mi> pmi;
typedef vector<mi> vmi;
typedef vector<pmi> vpmi;

/**
 * Description: 2D matrix operations. Change \texttt{d} 
 	* to array if possible.
 * Source: KACTL
 * Verification: https://dmoj.ca/problem/si17c1p5, SPOJ MIFF
 */

template<class T> struct Mat {
	int r,c; vector<vector<T>> d;
	Mat(int _r, int _c) : r(_r), c(_c) { 
		d.assign(r,vector<T>(c)); }
	Mat() : Mat(0,0) {} 
	Mat(const vector<vector<T>>&_d) : 
		r(sz(_d)), c(sz(_d[0])) { d = _d; }
	/// friend void pr(const Mat& m) { pr(m.d); }
	Mat& operator+=(const Mat& m) {
		/// assert(r == m.r && c == m.c);
		F0R(i,r) F0R(j,c) d[i][j] += m.d[i][j];
		return *this; }
	Mat& operator-=(const Mat& m) {
		/// assert(r == m.r && c == m.c);
		F0R(i,r) F0R(j,c) d[i][j] -= m.d[i][j];
		return *this; }
	Mat operator*(const Mat& m) {
		assert(c == m.r); Mat x(r,m.c);
		F0R(i,r) F0R(j,c) F0R(k,m.c) 
			x.d[i][k] += d[i][j]*m.d[j][k];
		return x; }
	Mat operator+(const Mat& m) { return Mat(*this)+=m; }
	Mat operator-(const Mat& m) { return Mat(*this)-=m; }
	Mat& operator*=(const Mat& m) { return *this = (*this)*m; }
	friend Mat pow(Mat m, ll p) {
		assert(m.r == m.c && p >= 0);
		Mat res(m.r,m.c); F0R(i,m.r) res.d[i][i] = 1;
		for (; p; p /= 2, m *= m) if (p&1) res *= m;
		return res;
	}
};

int main() {
	setIO();
	int N;
	ll K;
	cin >> N >> K;
	Mat<mi> a(N, N);
	
	for(int i = 0; i < N; i++){
		for(int j = 0; j < N; j++){
			int b;
			cin >> b;
			a.d[i][j] = b;
		}
	}
	a = pow(a, K);
	mi ans = 0;
	for(int i = 0; i < N; i++){
		for(int j = 0; j < N; j++){
			ans+=a.d[i][j];
		}
	}
	ps(ans);
}
 
/* stuff you should look for
	* int overflow, array bounds
	* special cases (n=1?), set tle
	* do smth instead of nothing and stay organized
*/