C++ :

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

int m, s, c,a[210],b[210];

bool cmp(int a, int b)
{
    return a > b;
}

int main()
{
    scanf("%d%d%d", &m, &s, &c);
    for (int i = 1; i <= c; i++)
        scanf("%d", &a[i]);
    sort(a + 1, a + c + 1);
    for (int i = 2; i <= c; i++)
        b[i] = a[i] - a[i - 1] - 1;
    int ans = a[c] - a[1] + 1;
    sort(b + 2, b + c + 2, cmp);
    for (int i = 1; i < m; i++)
        ans -= b[i + 1];
    printf("%d", ans);

    return 0;
}