#include <bits/stdc++.h>
using namespace std;
typedef long long LL;;
LL r, c, n;
vector<pair<LL,LL> > seeds;
const LL INF = 2100100100;
LL best = INF;
set<pair<LL,LL> > tried;
void tryxwithstart(LL x, LL startx){
if(x < 0 || tried.find({x,startx}) != tried.end()){
return;
}
LL endx = startx + r - 1;
// insert z.second at time z.first
// delete at time z.first + x + 1
map<LL, vector<pair<LL, LL> > > events;
for(pair<LL,LL> z : seeds){
if(z.first + x < startx || z.first > endx) continue;
events[max(z.first, startx)].push_back({0, z.second}); // ins
events[min(z.first + x, endx) + 1].push_back({1, z.second}); // del
}
LL maxup = 0;
LL maxdown = 0;
LL maxtot = 0;
multiset<LL> grass;
grass.insert(-1);
grass.insert(c);
multiset<LL> diffs;
diffs.insert(c+1);
for(auto x : events){
for(pair<LL,LL> event : x.second){
LL a = event.second;
if(event.first == 0){
grass.insert(a);
}
LL a1 = *(--grass.find(a));
LL a2 = *(++grass.find(a));
if(event.first == 0){
diffs.erase(diffs.find(a2-a1));
diffs.insert(a-a1);
diffs.insert(a2-a);
} else if(event.first == 1){
diffs.erase(diffs.find(a-a1));
diffs.erase(diffs.find(a2-a));
diffs.insert(a2-a1);
}
if(event.first == 1){
grass.erase(grass.find(a));
}
}
if(x.first > endx){
break;
}
maxtot = max(maxtot, *(--diffs.end()) );
maxup = max(maxup, *(++grass.begin()) - 0);
maxdown = max(maxdown, c-1 - *(--(--grass.end())) );
}
if(maxtot == c + 1){
return;
}
best = min(best, max(maxup + maxdown, maxtot - 1) + x);
}
void tryx(LL x){
vector<LL> stuff[r + x];
for(pair<LL,LL> z : seeds){
for(LL j = 0; j <= x; j++){
stuff[j + z.first].push_back(z.second);
}
}
vector<LL> up(r+x, 0);
vector<LL> down(r+x, 0);
vector<LL> tot(r+x, 0);
for(LL j = 0; j < r+x; j++){
vector<LL> grass = stuff[j];
sort(grass.begin(), grass.end());
if(grass.size() == 0){
up[j] = down[j] = tot[j] = INF;
} else {
up[j] = grass[0] - 0;
down[j] = c-1 - grass[grass.size()-1];
for(LL i = 0; i + 1 < grass.size(); i++){
tot[j] = max(tot[j], grass[i+1] - grass[i] - 1);
}
}
}
for(LL j = 0; j <= x; j++){
LL maxup = 0;
LL maxdown = 0;
LL maxtot = 0;
for(LL i = j; i < j + r; i++){
maxup = max(maxup, up[i]);
maxdown = max(maxdown, down[i]);
maxtot = max(maxtot, tot[i]);
}
best = min(best, max(maxup + maxdown, maxtot) + x);
}
}
int main(){
cin >> r >> c >> n;
for(LL i = 0; i < n; i++){
LL s, e;
cin >> s >> e;
s--; e--;
seeds.push_back({s,e});
}
sort(seeds.begin(), seeds.end());
if(r <= 40){
for(LL j = 0; j <= 2*r; j++){
tryx(j);
}
} else {
for(int i = 0; i < n; i++){
for(int j = 0; j < n; j++){
tryxwithstart(seeds[i].first + r-1 - seeds[j].first, seeds[i].first);
}
for(int j = 0; j < n; j++){
for(int k = 0; k < n; k++){
tryxwithstart(seeds[j].first - 1 - seeds[k].first, seeds[i].first);
}
}
}
}
cout << best << '\n';
}
