#include <stdio.h>

int main() {
    int n, first, last, temp;
    float median;
    
    scanf("%d", &n);
    
    if (n < 0) {
        printf("invalid Input");
        return 0;
    }
    
    last = n % 10;
    
    temp = n;
    while (temp >= 10) {
        temp = temp / 10;
    }
    first = temp;
    
    median = (first + last) / 2.0;
    
    printf("%.1f",median);
    
    return 0;
}
    
    
    
}