Contest1334/J.cpp

#include <bits/stdc++.h>
using namespace std;
int main(){
    int n, m;
    cin >> n >> m;
    int degree[n+1] = {0};
    int s, e;
    cin >> s >> e;
    for(int i = 1; i <= m; i++){
        int x, y;
        cin >> x >> y;
        degree[x]++;
        degree[y]++;
    }
    int a = 0, b = 0;
    int flag = 1;
    if(s == e){
        for(int i= 1; i <= n; i++){
            if(degree[i]%2==1){
                flag = 0;
                break;
            }
        }
    }else{
        for(int i = 1; i <= n; i++){
            if(i == s || i == e){
                if(degree[i] % 2 == 0){
                    flag = 0;
                    break;
                }
            }else{
                if(degree[i] % 2 == 1){
                    flag = 0;
                    break;
                }
            }
        }
    }
    if(flag){
        cout << "YES" << endl;
    }else{
        cout << "NO" << endl;
    }

}