#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int findLargestRobotSize(int n, int m, vector<vector<bool>>& grid) {
    vector<vector<int>> dp(n + 1, vector<int>(m + 1, 0));
    int maxSize = 0;

    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            if (!grid[i][j]) {  
                if (i == 1 || j == 1) {
                    dp[i][j] = 1;  
                } else {
                    dp[i][j] = min({dp[i-1][j], dp[i][j-1], dp[i-1][j-1]}) + 1;
                }
                maxSize = max(maxSize, dp[i][j]);
            }
        }
    }

    return (maxSize > 0) ? maxSize : -1;
}

int main() {
    int n, m, k;
    cin >> n >> m >> k;

    vector<vector<bool>> grid(n + 1, vector<bool>(m + 1, false));

    for (int i = 0; i < k; ++i) {
        int x, y;
        cin >> x >> y;
        grid[x][y] = true; 
    }

    int result = findLargestRobotSize(n, m, grid);
    cout << result << endl;

    return 0;
}