1 /****************************************************
  2
  3     Author : Coolxxx
  4     Copyright 2017 by Coolxxx. All rights reserved.
  5     BLOG : http://blog.csdn.net/u010568270
  6
  7 ****************************************************/
  8 #include<bits/stdc++.h>
  9 #pragma comment(linker,"/STACK:1024000000,1024000000")
 10 #define abs(a) ((a)>0?(a):(-(a)))
 11 #define lowbit(a) (a&(-a))
 12 #define sqr(a) ((a)*(a))
 13 #define mem(a,b) memset(a,b,sizeof(a))
 14 const double EPS=1e-8;
 15 const int J=10;
 16 const int MOD=100000007;
 17 const int MAX=0x7f7f7f7f;
 18 const double PI=3.14159265358979323;
 19 const int N=200004;
 20 using namespace std;
 21 typedef long long LL;
 22 double anss;
 23 LL aans;
 24 int cas,cass;
 25 int n,m,lll,ans;
 26 int q[N],last[N];
 27 int mark[N][2];
 28 bool u[N];
 29 struct xxx
 30 {
 31     int next,to,d;
 32 }a[N+N];
 33 void add(int x,int y,int z)
 34 {
 35     a[++lll].to=y;
 36     a[lll].d=z;
 37     a[lll].next=last[x];
 38     last[x]=lll;
 39 }
 40 bool spfa(int s)
 41 {
 42     int i,l=0,r=1,now,to,sumz=0,sumo=0;
 43     bool z=0,o=0;
 44     q[1]=s;mark[s][0]=0,mark[s][1]=1;u[s]=1;
 45     while(l!=r)
 46     {
 47         now=q[l=(l+1)%N];
 48         sumz+=mark[now][0];sumo+=mark[now][1];
 49         for(i=last[now];i;i=a[i].next)
 50         {
 51             to=a[i].to;
 52             if(mark[to][0]==-1)mark[to][0]=a[i].d-mark[now][0];
 53             if(mark[to][1]==-1)mark[to][1]=a[i].d-mark[now][1];
 54             if(mark[now][0]+mark[to][0]!=a[i].d || mark[to][0]<0 || mark[to][0]>1)z=1;
 55             if(mark[now][1]+mark[to][1]!=a[i].d || mark[to][1]<0 || mark[to][1]>1)o=1;
 56             if(!u[to])
 57             {
 58                 u[to]=1;
 59                 q[r=(r+1)%N]=to;
 60             }
 61         }
 62         if(z && o)break;
 63     }
 64     if(z && o)return 0;
 65     if(!z && o)ans+=sumz;
 66     else if(z && !o)ans+=sumo;
 67     else if(!z && !o)ans+=min(sumz,sumo);
 68     return 1;
 69 }
 70 int main()
 71 {
 72     #ifndef ONLINE_JUDGE
 73     freopen("1.txt","r",stdin);
 74 //  freopen("2.txt","w",stdout);
 75     #endif
 76     int i,j,k;
 77     int x,y,z;
 78 //  for(scanf("%d",&cass);cass;cass--)
 79 //  for(scanf("%d",&cas),cass=1;cass<=cas;cass++)
 80 //  while(~scanf("%s",s))
 81     while(~scanf("%d",&n))
 82     {
 83         mem(u,0);mem(mark,-1);
 84         scanf("%d",&m);
 85         for(i=1;i<=m;i++)
 86         {
 87             scanf("%d%d%d",&x,&y,&z);
 88             add(x,y,z);
 89             add(y,x,z);
 90         }
 91         ans=0;
 92         for(i=1;i<=n;i++)
 93         {
 94             if(u[i])continue;
 95             if(!spfa(i))break;
 96         }
 97         if(i<=n)puts("impossible");
 98         else printf("%d\n",ans);
 99     }
100     return 0;
101 }
102 /*
103 //
104
105 //
106 */