#include <iostream>
#include <fstream>
#define nmax 100005
using namespace std;

int n, k, v[nmax], sol = 0, sum = 0, scadAcum;

int main() {
	ifstream f("grupuri.in");
	ofstream g("grupuri.out");
	
	f>>k>>n;
	for(int i=1; i<=n; i++) f>>v[i], sum += v[i];

	for(int i=1; i<=n-k+1; i++)
		sol += v[i+1] - v[i];

	sol = (sum-sol) / k;

	g<<sol<<"\n";
	return 0;
}