#include <iostream>
using namespace std;
int main(){
    int math;
    int eng;
    int sci;
    int his;
    cin>>math>>eng>>sci>>his;
    float result;
    if(math<0&&math>100 || sci<0&&sci>100 ||his<0&&his>100 ||eng>100&&eng<0){
        cout<<"Invalid input";
    }
    else{
        result=math+sci+eng+his;
        cout<<fixed<<setprecision(2)<<result/4;
    }
    return 0;
}