#include <stdio.h>
int main(void){
    int p[2];
    int q[2];
    float dx = 0;
    float dy = 0;
    scanf( "%d %d", &p[0], &p[1] );
    scanf( "%d %d", &q[0], &q[1] );
    dx = p[0] - q[0];
    dy = p[1] - q[1];
    dx = ( dx < 0 ) ? dx * -1 : dx;
    dy = ( dy < 0 ) ? dy * -1 : dy;
    printf( "%.1f\n", dx / 2 + dy / 2 );
}