/*          ✿ ✿ ✿ بسم الله الرحمن الرحيـــم ✿ ✿ ✿
           اللهم _صل _وسلم _وبارك _علي _سيدنا _محمـــد

//****************************************************************************

    ░██████╗██╗░░██╗██████╗░░█████╗░██╗░░░██╗██╗░░██╗
    ██╔════╝██║░░██║██╔══██╗██╔══██╗██║░░░██║██║░██╔╝
    ╚█████╗░███████║██████╔╝██║░░██║██║░░░██║█████═╝░
    ░╚═══██╗██╔══██║██╔══██╗██║░░██║██║░░░██║██╔═██╗░
    ██████╔╝██║░░██║██║░░██║╚█████╔╝╚██████╔╝██║░╚██╗
    ╚═════╝░╚═╝░░╚═╝╚═╝░░╚═╝░╚════╝░░╚═════╝░╚═╝░░╚═╝
*/
//****************************************************************************
#include <bits/stdc++.h>
#define ll long long
#define pi  3.141592653
#define fixed(n) cout << fixed << setprecision(n)
#define getline(s) getline(cin >> ws , s)
#define Ceil(n , m) ((n / m) + (n % m ? 1 : 0))
#define Num_of_Digits(n) ((int)log10(n) + 1)
#define all(s) s.begin(), s.end()
#define rall(s) s.rbegin(), s.rend()
#define cin(v) for (auto&i:v) cin >> i;
#define cout(v) for (auto&i:v) cout << i << " ";
#define cin_2d(vec, n, m) \
for(int i=0;i<n;i++) for(int j=0;j<m&&cin>>vec[i][j];j++);
#define cout_2d(vec, n, m)\
for (int i = 0; i < n; i++, cout << "\n")\
for (int j = 0; j < m && cout << vec[i][j] << " "; j++)
#define cinp(v)  for (auto &i : v)  cin >> i.first >> i.second;
#define coutp(v)  for (auto &i : v) cout << i.first << " " << i.second << "\n";
#define sz(s) (ll)s.size()
#define mini(v) *min_element(all(v))
#define maxi(v) *max_element(all(v))
#define nl "\n"
#define EPS  1e-9
#define print(n) return void(cout<<n)
#define OO  2'000'000'000
using namespace std;
//****************************************************************************
void ShrOOk() {

 ios_base::sync_with_stdio(false),cin.tie(nullptr), cout.tie(nullptr);
    #ifndef ONLINE_JUDGE
    freopen("input.txt","r" , stdin),
    freopen("output.txt","w",stdout);
    #endif
}
//****************************************************************************

//*****************************************************************************
void solve()
{
   int n , m; cin >> n >> m;

   if(m > n)print(-1);

   vector < ll > v(n); cin(v);
   m--;
   int target = n - m;

   ll l = 0 , r = 0 , sum = 0 , mn = OO;
   while(r < n){

         while(r < n and (r - l) < target){
             sum += v[r++];
         }

         mn = min(mn , sum);
         sum -= v[l++];
   }

   cout << mn  ;
}
//****************************************************************************

int main()
{

   ShrOOk();
   int t=1;
    //  cin>>t;
   while(t--)
   {

       solve();
       cout<<nl;
   }


    return 0;
}

/*

*/