C :

#include <stdio.h>

int main()
{
    double p, q;
    while(scanf("%lf%lf", &p, &q)!=EOF){
        printf("%06lf\n", (p-q)/(p+q));
    }
    return 0;
}

C++ :

#include <cstdio>
#include <cstring>
#include <cstdlib>

int main(int argc, char **argv)
{
    int p, q;
    while(scanf("%d%d", &p, &q) == 2)
    {
        printf("%.6f\n", 1.0 * (p - q) / (p + q));
    }
    return 0;
}