#include <iostream>
#define llint long long

using namespace std;

llint n, k;
llint a[100005], b[100005];

bool check(llint x)
{
	llint cnt = 0;
	for(int i = 1; i <= n; i++){
		cnt += min(b[i]-a[i]+1, max(0LL, x-a[i]+1));
	}
	return cnt >= k;
}

int main(void)
{
	cin >> n >> k;
	for(int i = 1; i <= n; i++) cin >> a[i] >> b[i];
	
	llint ub = 1000000000, lb = 0, mid;
	while(ub-lb>1){
		mid = (ub+lb)/2;
		if(check(mid)) ub = mid;
		else lb = mid;
	}
	cout << ub << endl;
	
	return 0;
}