1.3.2Barn Repair

/*
ID: awsd1231
PROG: barn1
LANG: C++
*/
#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
int M, S, C, inC[300] = {0}, diffC[300] = {0};
int main() {
	freopen("barn1.in", "r", stdin);
	freopen("barn1.out", "w", stdout); 
	cin >> M >> S >> C;
	for(int i = 0; i != C; ++i) 
		cin >> inC[i];
	sort(inC, inC + C);
	for(int i = 0; i != C-1; ++i) {
		diffC[i] = inC[i+1] - inC[i];
	}
	sort(diffC, diffC + C-1);
	int sum = 0;
	int piece = M <= C-1 ? M-1 : C-1;
	for(int i = 0; i != piece; ++i) {
		sum += diffC[C-i-2] - 1;
	}
	sum += inC[0] - 1 + S - inC[C-1];
	sum = S - sum;
	cout << sum << endl;
	return 0;
}