squawk.cpp

/**
 *  @brief   Kattis - NAME 
 *  @author  Donald Dong (@donaldong)
 *  @date    MM/DD/YYYY
 *  
 *  + TAG
 */

#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <queue>
#include <regex>
#include <set>
#include <sstream>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;

typedef unsigned int uint;
typedef long long int ll;
typedef unsigned long long int ull;
typedef long double ld;
#define hmap unordered_map
#define hset unordered_set
#define pq priority_queue
#define pb push_back
#define mp make_pair
#define putchar putchar_unlocked
#define rep(i, s, e) for (size_t i = s, fe__ = e; i < fe__; ++i)

inline void scan(int&);
inline void scan(ll&);
inline void print(uint);
inline void print(ull);
inline void print(string&);

ll res;
int N, M, S, T;
vector<vector<bool>> G;
vector<ll> C;

void step() {
    res = 0;
    vector<ll> B = vector<ll>(C.size(), 0);
    rep(i, 0, N) {
        rep(j, 0, N) {
            if (G[i][j]) {
                B[j] += C[i];
                res += C[i];
            }
        }
    }
    C = B;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    scan(N); scan(M); scan(S); scan(T);
    G = vector<vector<bool>>(N, vector<bool>(N, false));
    C = vector<ll>(N, 0);
    C[S] = 1;
    rep(i, 0, M) {
        int u, v;
        scan(u); scan(v);
        G[u][v] = true;
        G[v][u] = true;
    }
    rep(i, 0, T) step();
    cout << res << endl;
    return 0;
}

inline void scan(int &number) {
    bool negative = false;
    int c;
    number = 0;
    c = getchar();
    if (c=='-') {
        negative = true;
        c = getchar();
    }
    for (; (c>47 && c<58); c=getchar()) number = number *10 + c - 48;
    if (negative) number *= -1;
}

inline void scan(ll &number) {
    bool negative = false;
    int c;
    number = 0;
    c = getchar();
    if (c=='-') {
        negative = true;
        c = getchar();
    }
    for (; (c>47 && c<58); c=getchar()) number = number *10 + c - 48;
    if (negative) number *= -1;
}

inline void print(uint n) {
    if (n == 0) {
        putchar('0');
        return;
    }
    char buf[11];
    int i = 10;
    while (n) {
        buf[i--] = n % 10 + '0';
        n /= 10;
    }
    while (i < 10) putchar(buf[++i]);
}

inline void print(ull n) {
    if (n == 0) {
        putchar('0');
        return;
    }
    char buf[20];
    int i = 19;
    while (n) {
        buf[i--] = n % 10 + '0';
        n /= 10;
    }
    while (i < 19) putchar(buf[++i]);
}

inline void print(string &s) {
    rep(i, 0, s.length()) putchar(s[i]);
}