#include <stdio.h>

int main(){
    int n;
    scanf("%d",&n);
    
    if (n<0){
        printf("Invalid input");
        return 0;
    }
    
    int last_digit >=n%10;
    int first_dight /=10;
    
    while (first dight >=10){
        first_dight/=10;
    }
    float median = (first_dight + last_dight)/2.0;
    
    printf("%,if", median);
    
    return 0;
}