#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.c < b.c;
}
struct Event {
bool toerase;
int l, c;
inline bool operator< (const Event &x) const {
return c < x.c || (c == x.c && toerase < x.toerase);
}
};
multiset<int> vert;
static inline int getAbove(int x, int r) {
multiset<int>::iterator it = vert.upper_bound(x);
if(it == vert.end())
return r + 1;
else
return *it;
}
static inline int getBelow(int x) {
multiset<int>::iterator it = vert.lower_bound(x);
if(it == vert.begin())
return 0;
else {
it--;
return *it;
}
}
long long getsol(int n, int r, int c) {
long long rez = 1LL << 35;
int necleft, necright;
necleft = necright = 1<<30;
for(int i = 0; i < n; ++i) {
necleft = min(necleft, v[i].c - 1);
necright = min(necright, c - v[i].c);
}
for(int i = 0; i < n; ++i)
for(int j = i; j < n; ++j) {
int expleft, expright;
long long auxrez;
expleft = necleft;
expright = max(necright, necleft + (v[j].c - v[i].c - 1 - necleft - necright));
auxrez = expleft + expright;
vector<Event> toadd, toerase;
vector<Event> events(2 * n);
for(int k = 0; k < n; ++k) {
toadd.push_back({0, v[k].l, max(1, v[k].c - expleft)});
toerase.push_back({1, v[k].l, min(c, v[k].c + expright)});
}
merge(toadd.begin(), toadd.end(), toerase.begin(), toerase.end(), events.begin());
int p1, p2;
int expup, expdown, middle;
expup = expdown = middle = 0;
p1 = 0;
for(int k = 0; k < n - 1; ++k)
if(v[k + 1].c - v[k].c - 1 > expleft + expright)
expup = expdown = middle = 1 << 30;
vert.clear();
while(p1 < events.size()) {
p2 = p1;
while(p2 < events.size() && events[p1].c == events[p2].c && !events[p2].toerase) {
vert.insert(events[p2].l);
++p2;
}
p2 = p1;
while(p2 < events.size() && events[p1].c == events[p2].c) {
int down, up;
up = getBelow(events[p2].l);
down = getAbove(events[p2].l, r);
if(up == 0)
expdown = max(expdown, events[p2].l - 1);
else
middle = max(middle, events[p2].l - up - 1);
if(down == r + 1)
expup = max(expup, r - events[p2].l);
else
middle = max(middle, up - events[p2].l - 1);
++p2;
}
p2 = p1;
while(p2 < events.size() && events[p1].c == events[p2].c) {
if(events[p2].toerase)
vert.erase(vert.find(events[p2].l));
++p2;
}
p1 = p2;
}
rez = min(auxrez + max(middle, expup + expdown), rez);
}
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;
}
