291C - Network Mask - CodeForces Solution

// LVJ submission #65af2abfd14038f289ef5963@1705978560317
#include<bits/stdc++.h>
#define gc getchar()
#define ll unsigned
using namespace std;
int n,k;
ll A,b,c,d,ans=-1;
ll a[100010];
map<ll,bool> mp;
int main() {
	cin>>n>>k;
	for(int i=1; i<=n; i++) {
		cin>>A,gc;
		cin>>b,gc;
		cin>>c,gc;
		cin>>d;
		a[i]=(A<<24)+(b<<16)+(c<<8)+d;
	}
	ll now=UINT_MAX;
	for(int i=1; i<=31; i++) {
		now-=(1<<i-1);
		int cnt=0;
		mp.clear();
		for(int j=1; j<=n; j++) if(!mp[now&a[j]]) mp[now&a[j]]=1,cnt++;
		if(cnt==k) ans=now;
	}
	if(ans==-1) cout<<"-1";
	else cout<<((ans>>24))<<"."<<((ans>>16)&255)<<"."<<((ans>>8)&255)<<"."<<(ans&255);
	return 0;
}