Timer Switch

#include<stdio.h>
typedef long long int ll;
ll Pr = 98765431;

int main(){
    int n;
    scanf("%d",&n);
    char str[n+1];
    scanf(" %s",str);

    ll ha = 0;
    ll hb = 0;

    ll mem = 1;
    for(int i = 0 ; i < n ; i ++){
        ha*=Pr;
        ha += (ll)str[i];
        if(i != 0)mem*=Pr;
    }
    hb = ha;
    for(int i = 0 ; i < n ; i ++){
        hb -= mem*(ll)str[i];
        hb *= Pr;
        hb += (ll)str[i];
        if(hb == ha){
            printf("%d",i+1);
            return 0;
        }
    }
    printf("error");
    return 0;
}