Find minimum value after first maximum value

Hello,

I got a simulink modell which outputs a curve like the red curve in the following picture.

http://upload.wikimedia.org/wikipedia/commons/thumb/e/ee/Sprungantwort_t2-schwingungsglied.png/688px-Sprungantwort_t2-schwingungsglied.png

Now I'm trying to measure the amplitude of this oscillation.

So I'll need to get the minimum after maximum value and subtract them.

How can I realise that the min aquisition starts right after the maximum found. In my case, the max finder always finds the right value and the min value is always around 0.

Can someone show me an algorithm which finds min after max.

An S-Function in C/C++ would be nice too. Thanks in advance.