(* Get input from user *)
n = Input["Enter initial number of nuclei: "]
tau = Input["Enter decay time constant tau: "]
dt = Input["Enter time step dt: "]
tMax = Input["Enter time to end simulation: "]

(* Initialize variables and output list *)
t = 0
result = {{t, n}}

(* Calculate results and store in list *)
While[t < tMax,
    n -= n / tau * dt;
    t += dt;
    result = Append[result, {t, n}]
]

(* Plot output and exact solution *)
euler = ListPlot[result, DisplayFunction->Identity]
exact = Plot[100 Exp[-t], {t, 0, 5}, DisplayFunction->Identity]
Show[euler, exact, DisplayFunction->$DisplayFunction]