#include <bits/stdc++.h>
using namespace std;
#define int long long

vector<vector<int>> dis = {{1, 0, -1, 0}, {0, 1, 0, -1}};
void dfs(int n, int m, vector<vector<bool>> &visited, vector<vector<int>> &ps, vector<vector<int>> &clean, int s, int i, int j) {
    visited[i][j] = true;
    clean[i][j]++; clean[i+s][j]--; clean[i][j+s]--; clean[i+s][j+s]++;
    for (int d = 0; d < 4; d++) {
        int i2 = i+dis[0][d], j2 = j+dis[1][d];
        if (1 <= i2 && i2 <= n-s+1 && 1 <= j2 && j2 <= m-s+1 && !visited[i2][j2] && ps[i2+s-1][j2+s-1]-ps[i2+s-1][j2-1]-ps[i2-1][j2+s-1]+ps[i2-1][j2-1] == 0) {
            dfs(n, m, visited, ps, clean, s, i2, j2);
        }
    }
}

bool works(int n, int m, vector<vector<bool>> &carpet, int s) {
    vector<vector<int>> ps(n+1, vector<int>(m+1, 0)), clean(n+2, vector<int>(m+2, 0));
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= m; j++) {
            ps[i][j] = ps[i-1][j] + ps[i][j-1] - ps[i-1][j-1] + carpet[i][j];
        }
    }

    for (int i = 1; i <= n-s+1; i++) {
        for (int j = 1; j <= m-s+1; j++) {
            if (ps[i+s-1][j+s-1]-ps[i+s-1][j-1]-ps[i-1][j+s-1]+ps[i-1][j-1] == 0) {
                vector<vector<bool>> visited(n+1, vector<bool>(m+1, false));
                dfs(n, m, visited, ps, clean, s, i, j);
                break;
            }
        }
    }

    vector<vector<int>> cps(n+1, vector<int>(m+1, 0));
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= m; j++) {
            cps[i][j] = cps[i-1][j] + cps[i][j-1] - cps[i-1][j-1] + clean[i][j];
            if (!carpet[i][j] && cps[i][j] == 0) return false;
        }
    }
    return true;
}

signed main() {
    // input
    int n, m, k; cin >> n >> m >> k;
    vector<vector<bool>> carpet(n+1, vector<bool>(m+1));
    for (int i = 0; i < k; i++) {
        int x, y; cin >> x >> y;
        carpet[x][y] = true;
    }

    // binary search
    int l = 0, r = min(n, m);
    while (l < r) {
        int s = (l+r+1)/2;
        if (works(n, m, carpet, s)) {
            l = s;
        } else {
            r = s-1;
        }
    }
    cout << (l == 0 ? -1 : l) << endl;
}