#include <stdio.h>
#include <string.h>
 
const int oo=(~0u)>>1;
const int MAXV=1102;
const int MAXE=6200;
typedef struct struct_edge* edge;
struct struct_edge{int v,c;edge n,b;};
struct_edge pool[MAXE];
edge top;
int S,T,V;
edge adj[MAXV];
int d[MAXV];
int q[MAXV];
int head,tail;
void add_edge(int u,int v,int c)
{
     top->v=v,top->c=c,top->n=adj[u],adj[u]=top++;
     top->v=u,top->c=0,top->n=adj[v],adj[v]=top++;
     adj[u]->b=adj[v],adj[v]->b=adj[u];
}
bool relabel()
{
     for (int i=0;i<V;d[i++]=oo) ;
     d[q[head=tail=0]=T]=0;
     while (head<=tail)
     {
           int u=q[head++];
           for (edge i=adj[u];i;i=i->n)
               if (i->b->c&&d[i->v]==oo)
                  d[q[++tail]=i->v]=d[u]+1;
           if (d[S]!=oo) return true;
     }
     return false;
}
int augment(int u,int e)
{
    if (u==T) return e;
    int f=0;
    for (edge i=adj[u];i&&e;i=i->n)
        if (i->c&&d[u]==d[i->v]+1)
           if (int df=augment(i->v,e<i->c?e:i->c))
              i->c-=df,i->b->c+=df,e-=df,f+=df;
    return f;
}
int dinic()
{
    int f=0;
    while (relabel()) f+=augment(S,oo);
    return f;
}
 
int N,M;
int ss,tt;
int total;
edge tt_to_ss;
bool read()
{
     total=0;
     top=pool;
     memset(adj,0,sizeof(adj));
     scanf("%d%d",&N,&M);
     if (N==0&&M==0) return false;
     ss=0,tt=N+1;
     S=N+2,T=N+3,V=N+4;
     for (int i=0;i<M;i++)
     {
         static const int MAXLENGTH=1024;
         char s[MAXLENGTH];
         int a,b,c;
         scanf("%s",&s);
         if (sscanf(s,"%d",&a)==EOF)
            if (*s=='+') a=ss;
            else if (*s=='-')  a=tt;
         scanf("%s",&s);
         if (sscanf(s,"%d",&b)==EOF)
            if (*s=='+') b=ss;
            else if (*s=='-') b=tt;
         scanf("%d",&c);
         add_edge(a,b,oo);
         add_edge(S,b,c);
         add_edge(a,T,c);
         total+=c;
     }
     add_edge(tt,ss,oo);
     tt_to_ss=top-2;
     return true;
}
void write()
{
     total-=dinic();
     if (total) printf("impossible\n");
     else
     {
         S=N+1,T=0,V-=2;
         int answer=tt_to_ss->b->c-dinic();
         printf("%d\n",answer);
     }
}
 
int main()
{
    while (read()) write();
}