def cerca_talla (stock, talla):
    for i in range (len(stock)):
        tallai, ni = stock[i]
        if talla == tallai:
            return i
    return -1

def modifica_talla (stock, pos, n):
    stock[pos][1]= stock[pos][1] + n

def modifica_stock(stock, entrades):
    for entrada in entrades:
        talla, n = entrada
        pos = cerca_talla(stock, talla)
        if pos != -1:
            modifica_talla(stock, pos, n)
        else:
            stock.append(entrada)