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f.py

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+from __future__ import print_function
+import sys
+
+N = int(sys.stdin.readline())
+K = int(sys.stdin.readline())
+
+D = {}; done = 0
+
+for line in sys.stdin:
+  C1, C2, P1, P2 = line.split(); C1, C2 = map(int, (C1, C2))
+  if P1 == P2: done += 1
+  D.setdefault(P1, set()).add(C1)
+  D.setdefault(P2, set()).add(C2)
+
+doubles = len([ k for k, v in D.iteritems() if len(v) == 2 ])
+singles = len([ k for k, v in D.iteritems() if len(v) == 1 ])
+
+left    = N - 2*doubles - singles
+answer  = doubles - done
+
+if singles == 0 and left == 2:
+  answer += 1
+elif singles == left:
+  answer += left
+
+print(answer)