include

include

using namespace std;

int n,m,d,a[1000005]; bool check(int x){

int prev=0;
int cnt=0;
for(int i=1;i<=n;i++){
	if(a[i]-a[prev]<x){
		cnt++;
		if(cnt>m){
			return false;
		}
	}
	else prev=i;
}
return true;

} int main() {

cin>>d>>n>>m;
for(int i=1;i<=n;i++){
	cin>>a[i];
}
a[0] = 0,a[n+1] = d;
sort(a+1,a+n+1);

int l=0,r=d;
while(l<r){
	int mid=l+r+1>>1;
	if(check(mid))l=mid;
	else r=mid-1;
}
cout<<l;
return 0;

}