I'm using KMP I don't know why I get TLE?



#include <iostream>



int main(int argc, const char * argv[]) {

    long long int NumofChars;
    long long int finAns=-1;

    scanf("%lld",&NumofChars);

    char lettersS[2*NumofChars];
    char lettersT[NumofChars];
    char letter_tmp;





    for (long long int i=0;i<NumofChars;i++)
    {
        scanf(" %c",&letter_tmp);
        lettersT[i]=letter_tmp;
    }

    for (long long int i=0;i<NumofChars;i++)
    {
        scanf(" %c",&letter_tmp);
        lettersS[i]=letter_tmp;
        lettersS[i+NumofChars]=letter_tmp;
    }




    long long int prefix[NumofChars];
    prefix[0]=0;
    prefix[1]=0;

    long long int pi=0;
    long long int pj=1;

    while(pj<NumofChars-1)
    {
        if(lettersT[pi]==lettersT[pj])
        {
            prefix[pj+1]=prefix[pj]+1;

            pi++;
            pj++;

        }
        else
        {

            pi=0;
            if(lettersT[pi]==lettersT[pj])
            {
                prefix[pj+1]=1;

                pi++;
                pj++;


            }
            else
            {
                prefix[pj+1]=0;

                pj++;

            }

        }
    }







    finAns=-1;

    long long int i=0;
    while(i<NumofChars)
    {


        long long int Matchcount=0;
        long long int j_init=0;
        for (long long int j=j_init;j<NumofChars;j++)
        {
            if(lettersS[i+j]==lettersT[j])
            {
                Matchcount++;
            }
            else
            {
                if(j-prefix[j]>0)
                {
                    i=i+j-prefix[j];
                    j_init=prefix[j];

                }
                else
                {
                    i++;
                }

                break;
            }
        }


        if(Matchcount==NumofChars)
        {
            finAns=i;

            break;
        }
    }
    std::cout<<finAns;
    return 0;
}