#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)(s); i < (int)(e); ++i)

int main() {
	cin.tie(nullptr);
	ios_base::sync_with_stdio(false);
	
	int N;
	ll K;
	cin >> N >> K;
	vector<ll> A(N), B(N);
	rep(i, 0, N) cin >> A[i] >> B[i];

	ll ng = 0, ok = 1ll << 40;
	while (ok - ng > 1) {
		ll mid = (ok + ng) / 2;
		
		ll cnt = 0;
		rep(i, 0, N) {
			if (mid < A[i]) continue;
			cnt += min(B[i] - A[i] + 1, mid - A[i] + 1);
			
			if (cnt >= K) break;
		}
		
		if (cnt >= K) ok = mid;
		else ng = mid;
	}
	cout << ok << '\n';
}