#include <cstdio>
#include <vector>
#include <queue>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <set>
#include <deque>
#include <numeric>
#include <map>
#define ll long long
using namespace std;
int N,K,ans,cnt;
int num=1;
deque<pair<int,int>> belt;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> N >> K;
for(int i=0;i<2*N;i++)
{
int a;
cin >> a;
belt.push_back({0,a});
}
while(true)
{
ans++;
auto tmp = belt.back(); belt.pop_back();
belt.push_front(tmp);
belt[0].first = 0;
belt[N-1].first = 0;
for(int i=N-2;i>=0;i--)
{
if(belt[i].first == 0) continue;
if(belt[i+1].first == 0 and belt[i+1].second > 0){
belt[i+1].second--;
belt[i+1].first = belt[i].first;
belt[i].first = 0;
}
}
if(belt[0].second > 0)
{
belt[0].first = num++;
belt[0].second--;
}
int cur_cnt = 0;
for(int i=0;i<2*N;i++)
if(belt[i].second == 0) cur_cnt++;
if(cur_cnt >= K) break;
}
cout << ans;
return 0;
}