#include <algorithm>
#include <iostream>
using ll = long long;
const int N = 300;
struct pt
{
int x, y;
} a[N];
struct el
{
ll a[N], b[N];
int f[N][N], l, r;
} p, q, c;
int v[N];
int main()
{
#define taskname "NGRASS"
if (fopen(taskname ".inp", "r"))
{
freopen(taskname ".inp", "r", stdin);
freopen(taskname ".out", "w", stdout);
}
int h, w, n, m;
ll ans = 1e18;
scanf("%d%d%d", &h, &w, &n);
for (int i = 0; i < n; i++)
scanf("%d%d", v + i, &a[i].x), a[i].y = v[i];
std::sort(v, v + n), v[m = std::unique(v, v + n) - v] = 1e9, std::sort(a, a + n, [](pt &x, pt &y)
{ return x.x < y.x; });
for (int i = 0; i < n; i++)
a[i].y = std::lower_bound(v, v + m, a[i].y) - v;
for (int l = 0, r, i, t; l < m; l++)
for (r = l; r < m; q.f[l][r++] = w - a[t].x)
for (i = 0, t = -1; i < n; i++)
if (l <= a[i].y && a[i].y <= r)
~t ? c.f[l][r] = std::max(c.f[l][r], a[i].x - a[t].x - 1) : p.f[l][r] = a[i].x - 1, t = i;
auto work = [&](int t)
{
int i = 0, j = 0;
ll k;
auto cl = [&](el &s)
{
s.l = 0, s.r = -1;
};
auto ad = [&](el &s)
{
int x = j < i ? s.f[j][i - 1] : 1e9;
while (s.l <= s.r && s.a[s.r] <= x)
s.r--;
s.a[++s.r] = x, s.b[s.r] = k;
};
auto qr = [&](el &s) -> ll
{
while (s.l <= s.r && s.b[s.l] <= k - h)
s.l++;
return s.a[s.l];
};
for (cl(p), cl(q), cl(c); i < m; i += v[i] == k, j += v[j] + t == k)
k = std::min(v[i], v[j] + t), ad(p), ad(q), ad(c),
ans = std::min(ans, std::max(qr(p) + qr(q), qr(c)) + t);
for (; j < m; j++)
k = v[j] + t, ad(p), ad(q), ad(c),
ans = std::min(ans, std::max(qr(p) + qr(q), qr(c)) + t);
};
for (int i = 0; i < m; i++)
for (int j = i; j < m; j++)
work(v[j] - v[i]), work(v[i] - v[j] + h), work(v[j] - v[i] + h);
printf("%lld", ans - 1);
}
