#pragma ide diagnostic ignored "misc-no-recursion"

#include "bits/stdc++.h"

using namespace std;
typedef long long ll;
typedef double db;
typedef complex<db> pt;

#define TESTCASE 0

static constexpr int MOD = 998'244'353;
static constexpr int INF = 1.1e9;

static int N, M, K;
static vector<pair<int, int>> C;
static vector<vector<int>> P1, P2;
static vector<vector<int>> vis;

static void reset(vector<vector<int>> &P) {
    for (auto &p: P) {
        fill(p.begin(), p.end(), 0);
    }
}

static void precalc(vector<vector<int>> &P) {
    for (int x = 0; x < N; x++) {
        for (int y = 0; y < M; y++) {
            if (x > 0 && y > 0) P[x][y] -= P[x - 1][y - 1];
            if (x > 0) P[x][y] += P[x - 1][y];
            if (y > 0) P[x][y] += P[x][y - 1];
        }
    }
}

static void add2(vector<vector<int>> &P, int x, int y, int v) {
    if (x < N && y < M) {
        P[max(x, 0)][max(y, 0)] += v;
    }
}

static void add(vector<vector<int>> &P, int x, int y, int l) {
    add2(P, x + 0, y + 0, 1);
    add2(P, x + 0, y + l, -1);
    add2(P, x + l, y + 0, -1);
    add2(P, x + l, y + l, 1);
}

static bool check(int L) {
    reset(P1);
    reset(P2);
    reset(vis);

    for (auto [x, y]: C) {
        add(P1, x - L + 1, y - L + 1, L);
    }

    precalc(P1);
    queue<pair<int, int>> Q;
    for (int x = 0; x < N; x++) {
        for (int y = 0; y < M; y++) {
            if (!P1[x][y]) {
                Q.emplace(x, y);
                break;
            }
        }
        if (!Q.empty()) break;
    }

    while (!Q.empty()) {
        auto [x, y] = Q.front();
        Q.pop();

        if (vis[x][y]) continue;
        vis[x][y] = true;
        if (P1[x][y]) continue;
        add(P2, x, y, L);

        array dx = {0, 0, 1, -1};
        array dy = {1, -1, 0, 0};
        for (int i = 0; i < 4; i++) {
            int nx = x + dx[i], ny = y + dy[i];
            if (nx >= 0 && nx < N && ny >= 0 && ny < M) {
                Q.emplace(nx, ny);
            }
        }
    }

    for (auto [x, y]: C) {
        add(P2, x, y, 1);
    }

    precalc(P2);
    int v = 0;
    for (int x = 0; x < N; x++) {
        for (int y = 0; y < M; y++) {
            v += !P2[x][y];
        }
    }

    return !v;
}

static void solve([[maybe_unused]] int tc) {
    cin >> N >> M >> K;

    vis.assign(N, vector<int>(M));
    P1.assign(N, vector<int>(M));
    P2.assign(N, vector<int>(M));

    for (int i = 0; i < K; i++) {
        int x, y;
        cin >> x >> y;
        C.emplace_back(x - 1, y - 1);
    }
    for (int x = 0; x < N; x++) {
        C.emplace_back(x, M);
    }
    for (int y = 0; y < M; y++) {
        C.emplace_back(N, y);
    }

    int l = 0, r = min(N, M) + 1;
    while (r - l > 1) {
        int m = (l + r) / 2;
        print(l, m, r);
        if (check(m)) {
            l = m;
        } else {
            r = m;
        }
    }

    cout << (l ? l : -1) << endl;
}

int main() {
    ios::sync_with_stdio(false);

    if (const char *f = getenv("REDIRECT_STDOUT"); f) {
        freopen(f, "w", stdout);
    }

    int T = 1;
#if TESTCASE
    cin >> T;
#endif

    for (int t = 1; t <= T; t++) {
        solve(t);
    }

    return 0;
}

answer.code: In function ‘void solve(int)’:
answer.code:126:9: error: ‘print’ was not declared in this scope; did you mean ‘rint’?
  126 |         print(l, m, r);
      |         ^~~~~
      |         rint
answer.code: In function ‘int main()’:
answer.code:141:16: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  141 |         freopen(f, "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~