| #The Biggest Water Problem#

#include <stdio.h>
int f(int n)
{
    while(1)
    {
        n=n%10+n/10;
        if(n>=10)
        f(n);
        else
        return n;
    }  
}

int main() 
{
    int n;
    scanf("%d",&n);
    printf("%d",f(n));
    return 0;
}