#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<LL> tried;
void tryxfast(LL x){
if(x < 0 || tried.find(x) != tried.end()){
return;
}
map<LL, vector<pair<LL, LL> > > events;
for(pair<LL,LL> z : seeds){
events[z.first].push_back({0, z.second}); // ins
events[z.first + x + 1].push_back({1, z.second}); // del
}
vector<LL> pos;
vector<LL> ups;
vector<LL> downs;
vector<LL> totals;
multiset<LL> grass;
grass.insert(-1);
grass.insert(c);
multiset<LL> diffs;
diffs.insert(c+1);
for(auto tt : events){
for(pair<LL,LL> event : tt.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));
}
}
pos.push_back(tt.first);
totals.push_back( *(--diffs.end()) );
ups.push_back( *(++grass.begin()) - 0 );
downs.push_back( c-1 - *(--(--grass.end())) );
}
for(int i = 0; i < totals.size(); i++){
if(totals[i] == c+1){
totals[i] = INF;
}
}
// two pointers:
int j = 0;
multiset<LL> upset;
multiset<LL> downset;
multiset<LL> totalset;
for(int i = 0; i < pos.size(); i++){
while(j < pos.size() && pos[j] <= pos[i] + r - 1){
upset.insert(ups[j]);
downset.insert(downs[j]);
totalset.insert(totals[j]);
j++;
}
best = min(best, max(*(--upset.end()) + *(--downset.end()), *(--totalset.end()) - 1) + x);
upset.erase(upset.find(ups[i]));
downset.erase(downset.find(downs[i]));
totalset.erase(totalset.find(totals[i]));
}
}
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++){
tryxfast(seeds[i].first + r-1 - seeds[j].first);
tryxfast(seeds[i].first - 1 - seeds[j].first);
}
}
cout << best << '\n';
}
