#include<bits/stdc++.h>
using namespace std;
int a[1005],b[1005],m,s,c,i,ans;
bool cmp(int a,int b){       //从大往小排
    return a>b;
}
int main(){
    //freopen("barn1.in","r",stdin);
    //freopen("barn1.out","w",stdout);
    cin>>m>>s>>c;
    for(i=1;i<=c;i++)
        cin>>a[i];
    sort(a+1,a+c+1);
    for(i=1;i<=c-1;i++) 
        b[i]=a[i+1]-a[i]-1;       //算Ci到Ci+1的距离
    sort(b+1,b+c,cmp);
    ans=a[c]-a[1]+1;             //ans最差情况，即赋初值
    for(i=1;i<m&&i<c;i++)
        ans-=b[i];                    //循环算答案
    cout<<ans;             //输出
    return 0;
}