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

using namespace std;

int largestRobotSize(int n, int m, vector<vector<int>>& obstructions) {
    vector<vector<int>> dp(n + 1, vector<int>(m + 1, 0));
    vector<vector<bool>> is_obstructed(n + 1, vector<bool>(m + 1, false));

    for (const auto& obs : obstructions) {
        int i = obs[0], j = obs[1];
        is_obstructed[i][j] = true;
    }

    int maxSize = 0;

    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            if (!is_obstructed[i][j]) {
                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<int>> obstructions(k, vector<int>(2));
    for (int i = 0; i < k; ++i) {
        cin >> obstructions[i][0] >> obstructions[i][1];
    }

    int result = largestRobotSize(n, m, obstructions);
    cout << result << endl;
    
    return 0;
}