#include <bits/stdc++.h>

#define MAXN 5050
#define pb push_back
#define INF 10000000

using namespace std;

typedef pair <int, int> pii;
typedef long long ll;

int main() {
    ofstream fout ("barn1.out");
    ifstream fin ("barn1.in");

    int m, s, c;
    fin >> m >> s >> c;

    priority_queue <int> fila;
    int stalls[MAXN];

    for(int i = 1; i <= c; i++){
        fin >> stalls[i];
    }

    sort(stalls + 1, stalls + c + 1);

    int anterior = stalls[1] + 1, atual;

    for(int i = 1; i <= c; i++){
        atual = stalls[i];
        fila.push(atual - anterior - 1);
        anterior = atual;
    }

    int sum = (stalls[c] - stalls[1]) + 1;

    for(int i = 1; i <= m - 1; i++){
        sum -= fila.top();
        fila.pop();
    }

    if(m > c) fout << c << endl;
    else fout << sum << endl;

    return 0;
}