话不多说

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
    int a;
    cin>>a;
    int b=a%10;
    a%=10;
    while(b>10)
    {
        a=b;
        b=a%10;
    }
    cout<<a<<endl;

    return 0;
}