from math import gcd

def prob(red_1, black_1, red_2, black_2):
    
    # Total outcomes
    total_1 = red_1 + black_1
    total_2 = red_2 + black_2
  
    total_outcomes = total_1 * (total_2 * (total_2 - 1) // 2)
    
    # Favorable outcomes
    # Black from Bag 1, 1 red + 1 black from Bag 2
    BRB = black_1 * (red_2 * black_2)   
    
    # Red from Bag 1, 2 blacks from Bag 2
    RBB = red_1 * (black_2 * (black_2 - 1) // 2)
    
    favorables = BRB + RBB
    
    divisor = gcd(favorables, total_outcomes)
    num = favorables // divisor
    den = total_outcomes // divisor
    
    return f"{num}/{den}"