#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned ll
#define ld long double
typedef vector<int> vi;
typedef multiset<int> mi;
typedef multiset<ll> mll;
typedef vector<ll> vll;
typedef vector<bool> vb;
typedef vector<string> vs;
typedef set<ll> sll;
typedef vector<vector<int>> _2vi;
typedef vector<vector<ll>> _2vll;
#define all(v) ((v).begin()), ((v).end())
#define sz(v) ((ll)((v).size()))
#define vinp(v, n) \
for (ull i = 0; i < (n); i++) \
cin >> (v)[i]
#define printv(v) \
for (auto i : (v)) \
cout << i << " "
#define fr0(i, n) for (ull(i) = 0; (i) < (n); (i)++)
#define fr1(i, n) for (ull(i) = 1; (i) < (n); (i)++)
#define fr(i, x, n) for (ull(i) = (x); (i) < (n); (i)++)
#define _CRT_SECURE_NO_WARNING
const ll MOD = 1000000007;
void Bustany() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
#ifndef ONLINE_JUDGE
freopen("./in.txt", "r", stdin), freopen("./out.txt", "w", stdout);
#endif
}
const ll N = 1e5 + 5;
vector<sll> adj(N);
//_2vll adj(N,vll(N));
vb vis;
ll n, m;
vll v;
bool can(ll time) {
vll x = v;
ll last = n - 1;
fr0(i, m) {
ll t = time;
while (last >= 0 && x[last] == 0) {
last--;
}
t -= last + 1;
if(t<=0){
break;
}
while(last>=0 && x[last]<=t){
t-=x[last];
last--;
}
if(last>=0){
x[last]-=t;
}
}
return last==-1;
}
void solve() {
cin >> n >> m;
v.assign(n, {});
vinp(v, n);
ll l = 1, r = 1e5 + 1e14, ans, mid;
while (l <= r) {
mid = (l + r) / 2;
if (can(mid)) {
r = mid - 1;
ans = mid;
} else {
l = mid + 1;
}
}
cout << ans;
}
int main() {
Bustany();
ll t = 1;
// cin >> t;
while (t--) {
solve();
}
}
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