// Skyqwq
#include <bits/stdc++.h>

#define pb push_back
#define fi first
#define se second
#define mp make_pair
#define int long long
using namespace std;

typedef long long LL;
typedef pair<int, int> PII;

template <typename T> void chkMax(T &x, T y) { if (y > x) x = y; }
template <typename T> void chkMin(T &x, T y) { if (y < x) x = y; }

template <typename T> void inline read(T &x) {
    int f = 1; x = 0; char s = getchar();
    while (s < '0' || s > '9') { if (s == '-') f = -1; s = getchar(); }
    while (s <= '9' && s >= '0') x = x * 10 + (s ^ 48), s = getchar();
    x *= f;
}

const int N = 305, INF = 1e9;

int X, Y, n, mn, d[N * N * 2], t, ans = 2e9, now;

PII a[N];

struct E{
	int x, op, A, B, C;
	bool operator < (const E &b) const { 
		if (b.x == x) return op > b.op;
		return x < b.x;
	}
} e[N << 1];

void inline build(int w) {
	int A = 0, B = 0, C = 0, x = e[w].x;
	int la = 0;
	for (int i = 1; i <= n; i++) {
		if (a[i].fi <= x && x <= a[i].fi + now) {
			if (la == 0) A = a[i].se - 1;
			else chkMax(C, a[i].se - la - 1);
			la = a[i].se;
		} 
	}
	B = Y - la;
	if (!la) A = B = INF;
	e[w].A = A, e[w].B = B, e[w].C = C;
}

struct Q{
	int q[N << 1], hh, tt, a[N << 1], v[N << 1], idx;
	void inline clear() { hh = 1, tt = idx = 0; }
	void inline ins(int x, int y) {
		a[++idx] = x, v[idx] = y;
		while (hh <= tt && v[q[tt]] <= y) tt--;
		q[++tt] = idx;
	}
	int inline get(int x) {
		while (hh <= tt && a[q[hh]] <= x - X) hh++;
		return v[q[hh]];
	}
} q[3];

void inline upd() {
	for (int j = 0; j < 3; j++) q[j].clear();
	for (int i = 1; i < 2 * n; i++) {
		if (e[i].x == e[i + 1].x) continue;
		int x = e[i + 1].x - 1;
		q[0].ins(x, e[i].A);
		q[1].ins(x, e[i].B);
		q[2].ins(x, e[i].C);
		if (e[i + 1].x != e[i].x && e[i + 1].x - e[1].x >= X) {
			int A = q[0].get(x);
			int B = q[1].get(x);
			int C = q[2].get(x);
			chkMin(ans, now + max(A + B, C));
		}
	}
}

bool inline cmp(PII x, PII y) {
	return x.se < y.se;
}

signed main() {
	read(X), read(Y), read(n);
	for (int i = 1; i <= n; i++) read(a[i].fi), read(a[i].se);
	sort(a + 1, a + 1 + n);
	mn = X - (a[n].fi - a[1].fi + 1);
	for (int i = 2; i <= n; i++)
		chkMax(mn, a[i].fi - a[i - 1].fi - 1);
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j < i; j++) {
			int v = a[i].fi - a[j].fi - 1;
			if (v >= mn) d[++t] = v;
		}
	}
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= n; j++) {
			int w = a[i].fi - 1 + X - a[j].fi;
			if (w >= mn) d[++t] = w;
		}
	}
	d[0] = mn;
	sort(d + 1, d + 1 + t);
	t = unique(d + 1, d + 1 + t) - d - 1;
	for (int i = 1; i <= n; i++) {
		e[i].x = a[i].fi;
		e[i + n].x = e[i].x + mn + 1;
		e[i + n].op = 1;
	}
	sort(e + 1, e + 1 + 2 * n);
	now = mn;
	sort(a + 1, a + 1 + n, cmp);
	for (int i = 1; i <= 2 * n; i++) build(i);
	upd();
	for (int i = 1; i <= t; i++) {
		int la = d[i] - d[i - 1]; now = d[i];
		for (int j = 1; j <= 2 * n; j++)
			if (e[j].op == 1) e[j].x += la;
		// sort(e + 1, e + 1 + 2 * n);
		// for (int j = 1; j <= 2 * n; j++) build(j);
		for (int j = 2; j <= 2 * n; j++) {
			int p = j;
			while (p > 1 && e[p] < e[p - 1]) {
				swap(e[p], e[p - 1]);
				build(p), build(p - 1);
				--p;
			}
		}
		upd();
	}
	printf("%lld\n", ans);
	return 0;
}