C++ :

#include<bits/stdc++.h>
using namespace std;
int n,m,j;
int main()
{
//  freopen("2.in","r",stdin);
//  freopen("2.out","w",stdout);
    cin>>n>>m>>j;
    int sum=0;  
    int l=1,r=l+m-1;
    for(int i=0;i<j;i++)
    {
        int x;
        cin>>x;   
        if(x>r) sum+=x-r,r=x,l=r-m+1;
        else if(x<l) sum+=l-x,l=x,r=l+m-1;
    }
    cout<<sum;
    return 0;
}