#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;

template<typename T>
ostream_iterator<T> oit(const string &s = " "){ return ostream_iterator<T>(cout,s.c_str()); }
inline auto rep(int l, int r) { return views::iota(min(l, r), r); }
inline auto rep(int n) { return rep(0, n); }
inline auto rep1(int l, int r) { return rep(l, r + 1); }
inline auto rep1(int n) { return rep(1, n + 1); }
inline auto per(int l, int r) { return rep(l, r) | views::reverse; }
inline auto per(int n) { return per(0, n); }
inline auto per1(int l, int r) { return per(l, r + 1); }
inline auto per1(int n) { return per(1, n + 1); }
#define A(a) begin(a),end(a)
inline auto len = ranges::ssize;

struct chash {
    static uint64_t splitmix64(uint64_t x) {
        // http://xorshift.di.unimi.it/splitmix64.c
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};
template<typename T, typename U> using pb_map = gp_hash_table<T, U, chash>;
template<typename T> using pb_set = gp_hash_table<T, null_type, chash>;
#define K first
#define V second

using ll = long long;
using ld = long double;

using vi = vector<int>;
using vii = vector<vector<int>>;
typedef vector<ll> vll;
using pll = pair<ll,ll>;
using pii = pair<int,int>;

constexpr ll NN = 2e5, M = 1000000007, L = 20;

const vector<pii> delta = {{1,0},{-1,0},{0,1},{0,-1}};

void run()
{
    int n,m,k; cin >> n >> m >> k;
    int ans = min(n,m);

    vii g(n,vi(m));
    for(int r,c; int _ : rep(k)){
        cin >> r >> c,--r,--c;
        g[r][c]=1;
    }

    { //impossible check
        int need = n*m - k;
        int have = 0;

        queue<pii> q;
        for(int i : rep(n)){
            bool found = false;
            for(int j : rep(m)){
                if(!g[i][j]){
                    q.emplace(i,j);
                    found=true;
                    break;
                }
            }
            if(found)break;
        }

        vii done(n,vi(m));
        while(not q.empty()){
            auto [r,c] {q.front()}; q.pop();
            if(done[r][c]) continue;
            have++,done[r][c]=true;
            for(int nr,nc; auto [dr,dc] : delta){
                nr = dr+r,nc=dc+c;
                if(nr<0 or nr>=n)continue;
                if(nc<0 or nc>=m)continue;
                if(done[nr][nc]) continue;
                if(g[nr][nc]) continue;
                q.emplace(nr,nc);
            }
        }
        if(need not_eq have)
            return void(cout << "-1\n");
    }

    //ans is the length of the min vertical or horizontal distance of 1 squares
    for(int i : rep(n)){
        int last = -1;
        for(int j : rep(m)){
            if(g[i][j]){
                int d = j-last-1;
                if(d) ans = min(ans,d);
                last=j;
            }
        }
        int d = m-last-1;
        if(d) ans = min(ans,d);
    }

    for(int j : rep(m)){
        int last = -1;
        for(int i : rep(n)){
            if(g[i][j]){
                int d = i-last-1;
                if(d) ans = min(ans,d);
                last=i;
            }
        }
        int d = n-last-1;
        if(d) ans = min(ans,d);
    }

    cout << ans << '\n';
}

int main()
{
    //KING OF THE WORLD...... U.W.T.B
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    run();
}