求助！！！
matlab SIR模型

function dy=odesir(t,y,beta,gamma)
dy=[-beta*y(1)*y(2);beta*y(1)*y(2)-gamma*y(2);gamma*y(2)];
N=100;beta=0.01;gamma=0.1;
tspan=[0:50];y0=[99,1,0];
[t,y]=ode45(@odesir,tspan,y0,[],beta,gamma);
plot(t,y);
xlabel('time');
ylabel('population');
legend('S','I','R')
end

运行不出来，总是提示“输入参数的数目不足
出错 odesir (line 2)
y=[-beta*y(1)*y(2);beta*y(1)*y(2)-gamma*y(2);gamma*y(2)];"