output_description: If there exists a set of values (x_1, x_2, ..., x_N) that is consistent with
  all given pieces of information, print `Yes`; if it does not exist, print
  `No`.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N M
      L_1 R_1 D_1
      L_2 R_2 D_2
      :
      L_M R_M D_M
attempt:  import sys                                                                                   
   l1 = list(map(int, input().split()))                                                         
   N = l1[0]                                                                                    
   M = l1[1]                                                                                    
                                                                                                
   dat = []                                                                                     
                                                                                                
   #per_dis = [[ 0 for n in range(N+1)] for n in range(N+1)]                                    
                                                                                                
   per_dis = [ [] for n in range(N+1)]                                                          
                                                                                                
   for n in range(M):                                                                           
       x = list(map(int, input().split()))                                                      
       dat.append(x)                                                                            
                                                                                                
                                                                                                
   for x in dat:                                                                                
       if per_dis[x[0]][x[1]] == 0 and per_dis[x[1]][x[0]] == 0:                                
           per_dis[x[0]][x[1]] = x[2]                                                           
           per_dis[x[1]][x[0]] = -x[2]                                                          
           for i in range(1,N):                                                                 
               for j in range(i+1,N):                                                           
                   if per_dis[i][j] != 0 and per_dis[i+1][j+1] != 0 and per_dis[i][j+1] == 0:   
                       per_dis[i][j+1] = per_dis[i][j] + per_dis[i+1][j+1]                      
                       per_dis[j+1][i] = -per_dis[i][j+1]                                       
                   elif per_dis[i][j] == 0 and per_dis[i+1][j+1] != 0 and per_dis[i][j+1] != 0: 
                       per_dis[i][j] = per_dis[i][j+1] - per_dis[i+1][j+1]                      
                       per_dis[j][i] = -per_dis[i][j]                                           
                   elif per_dis[i][j] != 0 and per_dis[i+1][j+1] == 0 and per_dis[i][j+1] != 0: 
                       per_dis[i+1][j+1] = per_dis[i][j+1] - per_dis[i][j]                      
                       per_dis[j+1][i+1] = -per_dis[i+1][j+1]                                   
       else:                                                                                    
           if abs(per_dis[x[0]][x[1]]) != x[2]:                                                 
               print("No")                                                                      
               sys.exit(0)                                                                      
                                                                                                
   print("Yes")                                                                                 
problem_description: Statement
  
  There are N people standing on the x-axis. Let the coordinate of Person i be
  x_i. For every i, x_i is an integer between 0 and 10^9 (inclusive). It is
  possible that more than one person is standing at the same coordinate.
  
  You will given M pieces of information regarding the positions of these
  people. The i-th piece of information has the form (L_i, R_i, D_i). This means
  that Person R_i is to the right of Person L_i by D_i units of distance, that
  is, x_{R_i} - x_{L_i} = D_i holds.
  
  It turns out that some of these M pieces of information may be incorrect.
  Determine if there exists a set of values (x_1, x_2, ..., x_N) that is
  consistent with the given pieces of information.
samples: [{"input": "3 3\n    1 2 1\n    2 3 1\n    1 3 2", "output": "Yes\n    \n\nSome possible sets of values (x_1, x_2, x_3) are (0, 1, 2) and (101, 102,\n103).\n\n* * *"}, {"input": "3 3\n    1 2 1\n    2 3 1\n    1 3 5", "output": "No\n    \n\nIf the first two pieces of information are correct, x_3 - x_1 = 2 holds, which\nis contradictory to the last piece of information.\n\n* * *"}, {"input": "4 3\n    2 1 1\n    2 3 5\n    3 4 2", "output": "Yes\n    \n\n* * *"}, {"input": "10 3\n    8 7 100\n    7 9 100\n    9 8 100", "output": "No\n    \n\n* * *"}, {"input": "100 0", "output": "Yes"}]