#include <bits/stdc++.h>
#define all(x) begin(x),end(x)

using namespace std;
using ll = long long;

const int N = 1e7 + 100;

bitset<N> bs;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int b, n;
    cin >> b >> n;
    vector<int> a(b);
    for (int i = 0; i < b; i++) cin >> a[i];
    for (int i = 0; i < N; i++) bs[i] = 1;
    bs[0] = 0;
    for (int val : a) {
        int pre = -10000;
        for (int t = bs._Find_first(); ; t = bs._Find_next(t)) {
            if (t - pre > n) {
                pre = t;
                bs[t] = 0;
                val--;
                if (val == 0) break;
            }
        }
    }
    int ans = 0;
    for (int t = 1; t < N; t++) {
        if (bs[t] == 0) ans = t;
    }
    cout << ans << '\n';
}