C++ :

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<algorithm>
using namespace std;
#define MAXN 100010
int n,m;
int a[MAXN];
inline int read(){
	int x=0,t=1,c;
	while(!isdigit(c=getchar())) if(c=='-') t=-1;
	while(isdigit(c)) x=x*10+c-'0',c=getchar();
	return x*t;
}
bool check(int x){
	int cnt=1,dis=a[1]+x;
	for(int i=2;i<=n;i++){
		if(a[i]<dis) continue;
		++cnt,dis=a[i]+x;
	}
	return cnt>=m;
}
void erfen(){
	int l=0,r=a[n]-a[1];
	while(l<=r){
		int mid=l+r>>1;
		if(check(mid)) l=mid+1;
		else r=mid-1;
	}
	printf("%d\n",r);
}
int main(){
	n=read(),m=read();
	for(int i=1;i<=n;i++) a[i]=read();
	sort(a+1,a+n+1);
	erfen();
	return 0;
}