C++ :

#include<bits/stdc++.h>
using namespace std;
int y,n,m,a[50005];
int main()
{
	cin>>y>>n>>m;
	for(int i=1;i<=n;i++)cin>>a[i];
	a[n+1]=y;
	int l=0,r=y+1;
	while(l+1<r)
	{
		int mid=(l+r)/2;
		int t=0,s=0;
		for(int i=1;i<=n+1;i++)
			if(a[i]-a[s]<mid)t++;
			else s=i;
		if(t>m)r=mid;
		else l=mid;
	}
	cout<<l;
	return 0;
}