#include <bits/stdc++.h>
using namespace std;
int n, m, r, j, a, ret;
int main(){
cin >> n >> m >> j;
int l = 1;
for(int i = 0; i < j; i++){
r = l + m - 1;
cin >> a;
if(a >= l && a <= r) continue;
else if(a < l){
l = a;
ret += l - a;
} else if(a > r){
l += a - r;
ret += a - r;
}
}
cout << ret << '\n';
}
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