How to fix this code? Sorry if it's a Vietnamese code

#include <iostream>
#include <cmath>
#include <string.h>
using namespace std;

int main(){
    double luythua,giatriluythua;
    cin >>luythua;
    giatriluythua = pow(1378,luythua);
    string so = to_string(giatriluythua);
    string chusocuoi = so[1];
    cout <<chusocuoi;
    return 0;

Why does it say that: error: conversion from ‘char’ to non-scalar type ‘std::string {aka std::basic_string}’ requested?
I am trying to print the last number of a number after powering it.

It’s better to use the % operator to get the last digit. Pulling out a character from a string and saving that to a string is messy.