#include <bits/stdc++.h>

#define FASTER() ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
#define ff first
#define ss second
#define pb push_back
#define all(a) a.begin(), a.end()
#define dbg(x) cerr<<" "<<#x<<" "<<x<<endl

typedef long long ll;

using namespace std;

int n, m;
vector <int> deg;

void fileIO() {
    freopen("gotham.in", "r", stdin);
    freopen("gotham.out", "w", stdout);
}

int check(int k) {
    int cur = 0;
    for(int i = 0; i < n; i++) {
        if(deg[i] >= k - 1) {
            cur++;
        }
    }
    return cur;
}

int main() {
    FASTER();
    fileIO();
    cin >> n >> m;
    ll sum = 0;
    deg.resize(n);
    for(int i = 0; i < n; i++) {
        cin >> deg[i];
        sum += deg[i];
    }
    if(m > 0 && n == 1) {
        cout << -1;
        return 0;
    }
    if(sum / 2 < m) {
        cout << -1;
        return 0;
    }
    int l = 1, r = n + 1;
    while(r - l > 1) {
        int mid = (r + l) >> 1;
        if(sum < mid) {
            r = mid;
            continue;
        }
        int cur = check(mid);
        dbg(mid);
        dbg(cur);
        if((cur - 1) * cur / 2 <= m && cur >= mid) { //satisfies
            l = mid;
        } else {
            r = mid;
        }
    }
    cout << l;
}