function dxdt = sniffer_ode(t,x,par,tu)

X = x(1);

R = x(2);


k1 = par(1);

k2 = par(2);

k3 = par(3);

k4 = par(4);


S = interp1(tu(:,1),tu(:,2),t);


dxdt(1) = k3*S-k4*X;

dxdt(2) = k1*S-k2*X*R;


dxdt = dxdt(:); %dxdt should be column

%sniffer
close all

%initial conditions:
X0=0; R0=0;
x0=[X0 R0];
%parameters:
k1=1; k2=1; k3=1; k4=1;
par=[k1 k2 k3 k4];
%input:
tu=[ 0   , 0
     1   , 0
     1.01, 1
    20   , 1];

[t,x] = ode45(@sniffer_ode,[0 20],x0, [],par,tu);

plot(t,x);