public int uniquePaths(int m, int n) {
        if (m <= 0 || n <= 0) return -1;
        int[] dp = new int[n];
        for (int j = 0; j < n; ++j) dp[j] = 1;

        for (int i = 1 ; i < m; ++i) {
            for (int j = 1; j < n; ++j) {
                dp[j] += dp[j - 1];
            }
        }
        return dp[n - 1];
    }