求助，20分

#include<bits/stdc++.h>
using namespace std;
int a,f[100000];
int main(){
	int n,m,i,j,l,ans=0;
	cin>>l>>n>>m;
	for(i=0;i<n;i++){
		int t=a;
		cin>>a;
		f[i]=a-t;
	}
	int h=l;
	int d=1;
	while(d<=h){
		int he=0,s=(h+d)/2;
		for(i=0;i<n;i++){
			if(f[i]<s){
				he++;
			}
		}
		if(he>m){
			h=s-1;
		}else{
			ans=s;
			d=s+1;
		}
		//cout<<d<<" "<<h<<endl;
	}
	cout<<ans;
	return 0;
}