Codeforces Round #540 (Div. 3) D. Coffee and Coursework(二分)

#include <bits/stdc++.h>
#define ll long long
using namespace std;
ll n,m;
ll pre[200005];

bool cmp(ll a,ll b){
  return a > b;
}

bool Check(ll x){
  ll ans = 0;
  for(int i=0;i<n;i++){
    ans += max(0LL, (ll)pre[i] - (i / x));
  }
  return ans >= m;
}

int main()
{
  scanf("%lld%lld",&n,&m);
  ll sum = 0;
  for(int i=0;i<n;i++){
    scanf("%lld",&pre[i]);
    sum += pre[i];
  }
  sort(pre, pre + n, cmp);
  if(sum < m){
    puts("-1");
    return 0;
  }
  ll l = 1, r = n, mid, ans = -1;
  while(l <= r){
    mid = (l + r) >> 1;
    if(Check(mid)){
      r = mid - 1;
      ans = mid;
    }
    else{
      l = mid + 1;
    }
  }
  cout<<ans<<endl;
  return 0;
}