>>> from sumatros import coincident

>>> coincident([2,3,5,5])
2
>>> coincident([0,3,5,5])
0
>>> coincident([10,3,5,5])
-1
>>> coincident([10,3,5,-5,13])
4
>>> coincident([10,3,5,-5,13,26])
4
>>> coincident([0,0,0,22])
0