#include<bits/stdc++.h>

#define fi first

#define se second

#define mp make_pair

#define pb push_back

#define pii pair<int,int>

#define C 0.5772156649

#define pi acos(-1.0)

#define ll long long

#define mod 1000000007

#define ls l,m,rt<<1

#define rs m+1,r,rt<<1|1

using namespace std;

const double g=10.0,eps=1e-;

const int N=+,maxn=+,inf=0x3f3f3f3f;

struct edge{

    int from,to,c,Next,low;

}e[maxn<<];

int cnt,head[N];

int dis[N];

int in[N],out[N];

void add(int u,int v,int c,int low)

{

    out[u]+=low;

    in[v]+=low;

    e[cnt].from=u;

    e[cnt].to=v;

    e[cnt].c=c;

    e[cnt].low=low;

    e[cnt].Next=head[u];

    head[u]=cnt++;

    e[cnt].from=v;

    e[cnt].to=u;

    e[cnt].c=;

    e[cnt].low=low;

    e[cnt].Next=head[v];

    head[v]=cnt++;

}

bool bfs(int s,int t)

{

    memset(dis,-,sizeof dis);

    dis[s]=;

    queue<int>q;

    q.push(s);

    while(!q.empty())

    {

        int x=q.front();

        q.pop();

        if(x==t)return ;

        for(int i=head[x];~i;i=e[i].Next)

        {

            int te=e[i].to;

            if(dis[te]==-&&e[i].c>)

            {

                dis[te]=dis[x]+;

                q.push(te);

            }

        }

    }

    return ;

}

int dfs(int x,int mx,int t)

{

    if(x==t)return mx;

    int flow=;

    for(int i=head[x];~i;i=e[i].Next)

    {

        int te=e[i].to,f;

        if(dis[te]==dis[x]+&&e[i].c>&&(f=dfs(te,min(mx-flow,e[i].c),t)))

        {

            e[i].c-=f;

            e[i^].c+=f;

            flow+=f;

        }

    }

    if(!flow)dis[x]=-;

    return flow;

}

int maxflow(int s,int t)

{

    int ans=,f;

    while(bfs(s,t))

    {

        while((f=dfs(s,inf,t)))ans+=f;

    }

    return ans;

}

void init()

{

    cnt=;

    memset(head,-,sizeof head);

    memset(in,,sizeof in);

    memset(out,,sizeof out);

}

int change(string a,int s,int t)

{

    if(a=="+")return s;

    else if(a=="-")return t;

    else

    {

        int ans=;

        for(int i=;i<a.size();i++)

            ans=ans*+a[i]-'';

        return ans;

    }

}

int main()

{

    ios::sync_with_stdio(false);

    cin.tie();

    int n,m;

    while(cin>>n>>m)

    {

        if(!n&&!m)break;

        init();

        int s=n+m+,t=n+m+;

        for(int i=;i<m;i++)

        {

            string a,b;

            int c;

            cin>>a>>b>>c;

            add(change(a,s,t),change(b,s,t),inf-c,c);

        }

        int ss=n+m+,tt=n+m+,sum=;

        for(int i=;i<=n+m+;i++)

        {

            if(in[i]>out[i])add(ss,i,in[i]-out[i],),sum+=in[i]-out[i];

            else add(i,tt,out[i]-in[i],);

        }

        int flow=maxflow(ss,tt);

        add(t,s,inf,);

        flow+=maxflow(ss,tt);

        if(flow!=sum)cout<<"impossible"<<endl;

        else cout<<e[cnt-].c<<endl;

    }

    return ;

}

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