Problem C: Magic Checkerboard

#include <bits/stdc++.h>

using namespace std;

const long long oo = (1e18) + 7;

int main()
{
    freopen("test.inp","r",stdin);
    int n, m;
    cin>>n>>m;
    int a[n][m];
    for (int i = 0;i<n;++i)
    for (int j = 0;j<m;++j) cin>>a[i][j];

    ////////////////////

    long long res = oo;
    int t[2][2];
    int mat[n][m];

    for (int i = 0;i<2;++i){
        t[0][0] = i;
        t[1][1] = 1-i;
        for (int j = 0;j<2;++j){
            t[1][0] = j;
            t[0][1] = 1-j;
            for (int r = 0;r<n;++r)
                for (int c = 0;c<m;++c)
                {
                    mat[r][c] = a[r][c];
                }

                // Solve
                long long cnt = 0;
                bool isConnected = (n > 1 && m > 1);
                bool isRunning = true;
                for (int r = 0;r<n;++r){
                    for (int c = 0;c<m;++c){
                        int prevCol = 0, prevRow = 0;
                        if (r>0) prevRow = mat[r-1][c];
                        if (c>0) prevCol = mat[r][c-1];
                        int NextValue = max(prevCol, prevRow) + 1;
                        if (isConnected && NextValue %2 != t[r%2][c%2]) NextValue++;

                        if (mat[r][c] == 0)
                            mat[r][c] = NextValue;
                        else if ((mat[r][c] < NextValue) || (isConnected && mat[r][c] % 2 != NextValue % 2))
                            {
                                isRunning = false;
                                break;
                            }
                        cnt += mat[r][c];
                    }
                    if (isRunning == false) {cnt = oo; break;}
                }
                // endl
                res = min(res, cnt);
        }
    }
    if (res == oo) cout << -1; else
    cout << res;
}