You need to remove no more than 256 numbers. Let's calculate dp[N][K], where N — current Index, K — count of deleted numbers.

dp[i+1][j] = max (dp[i+1][j], dp[i][j] + (a[i]^(i-j)));
dp[i+1][j+1] = max (dp[i+1][j+1], dp[i][j]);