import sys

def main():
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx]); idx += 1
    M = int(input[idx]); idx += 1
    black = set()
    for _ in range(M):
        r = int(input[idx]) - 1
        c = int(input[idx + 1]) - 1
        black.add((r, c))
        idx += 2

    min_x = N * N  # Initialize with maximum possible X

    # Precompute T matrix: T[i][j] = 1 if (i,j) is white, else 0
    # Instead of a full matrix, use a function to check on the fly
    def is_black(i, j):
        return (i, j) in black

    # Try each column j as the base column
    for j in range(N):
        for c_j in [0, 1]:
            # Determine R[i] based on column j and c_j
            R = [0] * N
            for i in range(N):
                T_ij = 0 if is_black(i, j) else 1
                R[i] = T_ij ^ c_j

            # Determine C[k] for other columns
            C = [0] * N
            C[j] = c_j  # Fixed base column

            for k in range(N):
                if k == j:
                    continue
                cnt0 = 0
                cnt1 = 0
                for i in range(N):
                    T_ik = 0 if is_black(i, k) else 1
                    expected_c = R[i] ^ T_ik
                    if expected_c == 0:
                        cnt0 += 1
                    else:
                        cnt1 += 1
                if cnt0 >= cnt1:
                    C[k] = 0
                else:
                    C[k] = 1

            # Calculate the number of valid cells
            valid = 0
            for i in range(N):
                for k in range(N):
                    T_ik = 0 if is_black(i, k) else 1
                    if (R[i] ^ C[k]) == T_ik:
                        valid += 1
            current_x = N * N - valid
            if current_x < min_x:
                min_x = current_x

    print(min_x)

if __name__ == '__main__':
    main()