#include "bits/stdc++.h"
using namespace std;
#define MAX 2002
int n;
int r;
int c;
vector<pair<long long int, long long int> > v;
vector<long long int> vv;
struct st {
map<long long int, int> mp;
priority_queue<pair<long long int, long long int> > q;
long long int las_up;
long long int las_dw;
long long int las_md;
bool changu;
bool changd;
bool changm;
st() {
changu = changd=changm=true;
}
void ins(long long int val) {
mp[val]++;
changu = changd = changm = true;
auto it = mp.lower_bound(val);
if (next(it) != mp.end()) {
q.push(make_pair((*next(it)).first - (*it).first, (*it).first));
}
if (it != mp.begin()) {
q.push(make_pair(-(*prev(it)).first + (*it).first, (*prev(it)).first));
}
}
void er(long long int val) {
mp[val]--;
changu = changd = changm = true;
if (mp[val] == 0) {
mp.erase(val);
auto it = mp.lower_bound(val);
if (it != mp.begin() && it != mp.end()) {
q.push(make_pair(-(*prev(it)).first + (*it).first, (*prev(it)).first));
}
}
}
long long int up() {
if (changu == false) {
return las_up;
}
changu = false;
if (mp.size() == 0) {
las_up = LLONG_MAX / 100LL;
return LLONG_MAX / 100LL;
}
las_up = (*mp.begin()).first;
return las_up;
}
long long int dw() {
if (changd == false) {
return las_dw;
}
changd = false;
if (mp.size() == 0) {
las_dw = LLONG_MAX / 100LL;
return LLONG_MAX / 100LL;
}
las_dw= r - (*mp.rbegin()).first - 1LL;
return las_dw;
}
long long int mid() {
if (changm == false) {
return las_md;
}
changm = false;
while (!q.empty()) {
long long int n1 = q.top().second;
long long int n2 = q.top().first + n1;
if (mp.count(n1) == 0 || mp.count(n2) == 0) {
q.pop();
continue;
}
auto it = mp.upper_bound(n1);
if ((*it).first != n2) {
q.pop();
continue;
}
las_md=n2 - n1 - 1LL;
return las_md;
}
las_md = 0;
return las_md;
}
};
st z[2002];
vector<long long int> sear;
struct monotonic_queue {
pair<long long int, int> v[2002];
int fr;
int bc;
monotonic_queue() {
fr = 0;
bc = -1;
}
void monotonic_queue1() {
fr = 0;
bc = -1;
}
void add(pair<long long int, int> p) {
while (bc>=fr&&v[bc].first <= p.first) {
bc--;
}
v[bc + 1] = p;
bc++;
}
long long int gt(long long int ic) {
while (sear[v[fr].second] >= ic + c) {
fr++;
}
return v[fr].first;
}
};
vector<long long int> mg(vector<long long int> &a, vector<long long int> &b) {
vector<long long int> r;
int aa = 0;
int bb = 0;
while (aa < a.size() || bb < b.size()) {
if (bb<b.size() && r.size() && r.back() == b[bb]) {
bb++;
continue;
}
if (aa<a.size() && r.size() && r.back() == a[aa]) {
aa++;
continue;
}
if (bb<b.size() && (aa == a.size() || a[aa]>b[bb])) {
r.push_back(b[bb]);
bb++;
continue;
}
r.push_back(a[aa]);
aa++;
continue;
}
return r;
}
bool cmp(pair<long long int, long long int> a, pair<long long int, long long int> b) {
return a.second < b.second;
}
vector<int> las;
long long int u1[MAX];
long long int d1[MAX];
long long int md1[MAX];
monotonic_queue u2;
monotonic_queue d2;
monotonic_queue md2;
int main() {
scanf("%d%d", &r, &c);
cin >> n;
long long int mint = LLONG_MAX;
long long int maxt = 0;
for (int i = 0; i < n; i++) {
int a, b;
scanf("%d%d", &a, &b);
a--;
b--;
v.push_back(make_pair(a, b));
mint = min(mint, (long long int)b);
maxt = max(maxt, (long long int)b);
}
for (int i = 0; i < v.size(); i++) {
sear.push_back(v[i].second + c);
sear.push_back(v[i].second);
sear.push_back(v[i].second - c);
}
sort(sear.begin(), sear.end());
sear.erase(unique(sear.begin(), sear.end()), sear.end());
maxt = c - maxt - 1LL;
long long int lef_least = mint;
long long int rig_least = maxt;
maxt += mint;
vv.push_back(0);
v.push_back(make_pair(0, c - 1));
v.push_back(make_pair(0, 0));
for (int i = 0; i < v.size(); i++) {
for (int j = 0; j < v.size(); j++) {
vv.push_back(v[i].second + c - v[j].second - 1);
vv.push_back(max(0LL, abs(v[j].second - v[i].second) - 1));
}
}
v.pop_back();
v.pop_back();
vv.push_back(maxt);
sort(vv.begin(), vv.end());
vv.erase(unique(vv.begin(), vv.end()), vv.end());
long long int ans = LLONG_MAX;
sort(v.begin(), v.end(), cmp);
for (int j = 0; j < v.size(); j++) {
int id1 = lower_bound(sear.begin(), sear.end(), v[j].second) - sear.begin();
id1--;
las.push_back(id1);
}
for (int i = 0; i < vv.size(); i++) {
if (vv[i] < maxt)continue;
for (int j = 0; j < v.size(); j++) {
for (int k = las[j] + 1;k+1<sear.size()&&sear[k+1]<=v[j].second+vv[i]+1LL; k++) {
las[j] = k;
z[k].ins(v[j].first);
}
}
for (int j = 0; j < sear.size(); j++) {
u1[j] = z[j].up();
d1[j]= z[j].dw();
md1[j] = z[j].mid();
}
u2.monotonic_queue1();
d2.monotonic_queue1();
md2.monotonic_queue1();
int idx = sear.size() - 1;
for (int j = sear.size() - 1; j >= 0; j--) {
u2.add(make_pair(u1[j], j));
d2.add(make_pair(d1[j], j));
md2.add(make_pair(md1[j], j));
while (sear[idx] > sear[j] + c) {
idx--;
}
if (sear[idx]==sear[j]+c) {
long long int need = u2.gt(sear[j]) + d2.gt(sear[j]) + max(0LL, md2.gt(sear[j]) - u2.gt(sear[j]) - d2.gt(sear[j]));
ans = min(ans, need + vv[i]);
}
}
}
printf("%lld\n", ans);
return 0;
}
