#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <set>
#include <unordered_set>
#include <random>
#include <ctime>
#include <queue>
#include <utility>

int nn, mm;
int MAXN = 1e6 * 2;


int IndexToNumber(int ii, int jj) {
    return ii * mm + jj;
}

int BitByIndex(int table, int ii, int jj) {
    return (table >> IndexToNumber(ii, jj)) % 2;
}

bool Check(int table) {
    for (int ii = 0; ii < nn; ++ii) {
        for (int jj = 0; jj < mm; ++jj) {
            if (ii > 0) {
                if (BitByIndex(table, ii, jj) == BitByIndex(table, ii - 1, jj)) {
                    return false;
                }
            }
            if (jj > 0) {
                if (BitByIndex(table, ii, jj) == BitByIndex(table, ii, jj - 1)) {
                    return false;
                }
            }
        }
    }
    return true;
}


int XorByIndex(int table, int ii, int jj) {
    return table xor (1 << IndexToNumber(ii, jj));
}

void PushNew(std::vector<bool> &used, std::queue<std::pair<int, int>> &qu, int table, int num) {
    for (int ii = 0; ii < nn; ++ii) {
        for (int jj = 0; jj < mm; ++jj) {
            if (ii + 1 != nn) {
                int new_table = XorByIndex(table, ii, jj);
                new_table = XorByIndex(new_table, ii + 1, jj);

                if (!used[new_table]) {
                    used[new_table] = true;
                    qu.push({new_table, num + 1});
                }
            }

            if (jj + 1 != mm) {
                int new_table = XorByIndex(table, ii, jj);
                new_table = XorByIndex(new_table, ii, jj + 1);

                if (!used[new_table]) {
                    used[new_table] = true;
                    qu.push({new_table, num + 1});
                }
            }
        }
    }
}

int Bfs(int table) {
    std::vector<bool> used(MAXN);
    std::queue<std::pair<int, int>> qu;
    qu.push({table, 0});
    used[table] = true;
    while (!qu.empty()) {
        auto item = qu.front();
        if (Check(item.first)) {
            return item.second;
        }
        qu.pop();
        PushNew(used, qu, item.first, item.second);
    }
    return -1;
}

int main() {
    std::cin >> nn >> mm;
    int table = 0;
    for (int i = 0; i < nn; ++i) {
        for (int j = 0; j < mm; ++j) {
            char sim;
            std::cin >> sim;
            if (sim == '1') {
                table = table | (1 << (i * mm + j));
            }
        }
    }
    std::cout << Bfs(table);
}