#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

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

    cout << [&]() {
        ll ng = 0, ok = 1e18;
        while(ok - ng > 1) {
            ll mid = (ok + ng) / 2;
            ([&](ll x) {
                ll cnt = 0;
                rep(i,N) cnt += max(0LL , min(x, B[i]) - A[i] + 1);
                return cnt >= K;
            }(mid) ? ok : ng) = mid;
        }
        return ok;
    }() << endl;
}