#include <stdio.h>
int main()
{
    int n;
    scanf("%d",&n);
    
    if(n<0){
        printf("invalid input\n");
        return 0;
        
    }
    int last_digit=n%10;
    int first_digit=n;
    while(first_digit>=10){
        first_digit/=10;
    }
    float median+(first_digit+last_digit)/2.0;
    printf("%1f\n",median);
    return 0;
}