python.graphs

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n, m = map(int, input().split())
a = [[0 for i in range(n)] for j in range(n)]
 
for i in range(m):
    u, v = map(int, input().split())
    u -= 1
    v -= 1
    a[u][v] = 1
    a[v][u] = 1
 
res = True
for i in range(n):
    for j in range(i + 1, n):
        for k in range(n):
            if ((i != k) and (j != k) and (a[i][j] == 1) and 
                (a[j][k] == 1) and (a[i][k] == 0)):
                res = False
 
if res:
    print('YES')
else:
    print('NO')			

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