Highest Value Palindrome

// Complete the highestValuePalindrome function below.
    static string highestValuePalindrome(string s, int n, int k) {
        int start=0,end = s.Length-1;
        int possible = -1;
        for(int i=0;i<(s.Length/2);i++){
            if(k>=2 && start<end){
                //start end diffrent
                if(s[start]!=s[end]){
                    if(s[start]!='9'){
                        k-=2;
                    }else{
                        k-=1;
                    }
                    s[start]!='9';
                    s[end] = '9';
                    start++;
                    end--;
                }else{
                    start++;
                    end--;
                }
            }
        }
    }