Count pairs Sum in matrices

import java.util.*;

class Solution {
    int countPairs(int[][] mat1, int[][] mat2, int x) {
        HashSet<Integer> set = new HashSet<>();
        for (int i = 0; i < mat2.length; i++) {
            for (int j = 0; j < mat2[0].length; j++) {
                set.add(mat2[i][j]);
            }
        }
        int count = 0;
        for (int i = 0; i < mat1.length; i++) {
            for (int j = 0; j < mat1[0].length; j++) {
                if (set.contains(x - mat1[i][j]))
                    count++;
            }
        }
        return count;
    }
}