#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 300;
struct Cell {
int l, c;
}v[MAX_N];
bool cmp(Cell a, Cell b) {
return a.l < b.l;
}
struct Range {
int l, r, dep;
inline bool operator< (const Range &x) const {
return l < x.l;
}
};
int dif;
set<Range> ranges;
int expup, expdown, middle;
void insertRange(int l, int r, int dep) {
set<Range>::iterator it, it2;
vector<Range> toerase, toadd;
it = ranges.lower_bound({l, 0, 0});
if(it != ranges.begin()) {
it2 = it;
it2--;
if(it2->r >= l) {
if(it2->dep == 0)
expup = max(expup, dep - it2->dep - 1);
else
middle = max(middle, dep - it2->dep - 1);
toerase.push_back(*it2);
if(it2->l < l)
toadd.push_back({it2->l, l - 1, it2->dep});
if(it2->r > r)
toadd.push_back({r + 1, it2->r, it2->dep});
}
}
while(it != ranges.end() && it->l <= r) {
toerase.push_back(*it);
if(it->dep == 0)
expup = max(expup, dep - it->dep - 1);
else
middle = max(middle, dep - it->dep - 1);
if(it->r > r)
toadd.push_back({r + 1, it->r, it->dep});
it++;
}
toadd.push_back({l, r, dep});
for(auto it: toerase)
ranges.erase(it);
for(auto it: toadd)
ranges.insert(it);
}
long long expand(int n, int r, int c, int expleft, int expright) {
ranges.clear();
ranges.insert({1, c, 0});
expup = expdown = middle = 0;
for(int i = 0; i < n; ++i) {
insertRange(max(1, v[i].c - expleft), min(c, v[i].c + expright), v[i].l);
}
for(auto it: ranges)
if(it.dep == 0) // empty gap
return 1LL << 35;
else
expdown = max(expdown, r - it.dep);
return expleft + expright + max(middle, expup + expdown);
}
long long getsol(int n, int r, int c) {
int necleft, necright;
long long rez = 1LL<<35;
necleft = necright = 1000000001;
for(int i = 0; i < n; ++i) {
necleft = min(necleft, v[i].c - 1);
necright = min(necright, c - v[i].c);
}
for(int expleft = necleft; expleft <= c; ++expleft)
for(int expright = necright; expright <= c; ++expright)
rez = min(rez, expand(n, r, c, expleft, expright));
return rez;
}
int main() {
#ifdef HOME
FILE *fin = fopen("input.in", "r");
FILE *fout = fopen("output.out", "w");
#else
FILE *fin = stdin;
FILE *fout = stdout;
#endif
int n, r, c;
fscanf(fin, "%d%d", &r, &c);
fscanf(fin, "%d", &n);
for(int i = 0; i < n; ++i)
fscanf(fin, "%d%d", &v[i].l, &v[i].c);
sort(v, v + n, cmp);
fprintf(fout, "%lld", getsol(n, r, c));
fclose(fin);
fclose(fout);
return 0;
}
