#include <stdio.h>

int main(){
	double val[4];
	for(int i = 0;i < 4;i ++){
		scanf("%lf",&val[i]);
		if(val[i] < 0){
			val[i] *= -1;
		}
	}
	printf("%lf",(val[0] + val[2]) + (val[1] + val[3]) / 2);
}