#include<bits/stdc++.h>
using namespace std;
 
#define debug(x) cerr << #x << ": " << (x) << endl
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define dwn(i,a,b) for(int i=(a);i>=(b);i--)
#define pb push_back
#define all(x) (x).begin(), (x).end()
 
#define x first
#define y second
using pii = pair<int, int>;
using ll = long long;
 
#define int ll
 
inline void read(int &x){
    int s=0; x=1;
    char ch=getchar();
    while(ch<'0' || ch>'9') {if(ch=='-')x=-1;ch=getchar();}
    while(ch>='0' && ch<='9') s=(s<<3)+(s<<1)+ch-'0',ch=getchar();
    x*=s;
}

const int N=110, mod=1e9+7;

int n, m;
int f[N][N];
int col[N];

int fpow(int x, int p){
	int res=1;
	for(; p; p>>=1, x=1LL*x*x%mod) if(p&1) res=1LL*res*x%mod;
	return res%mod;
}

int inv(int x){
	return fpow(x, mod-2);
}

int add(int x, int y){
	return (x+y+mod)%mod;
}

int mul(int x, int y){
	return x*y%mod;
}

int dv(int x, int y){
	return mul(x, inv(y));
}

map<string, int> mp;
int get(string &s){
	if(mp.count(s)) return mp[s];
	return mp[s]=++m;
}

// parameter?
bool par(string &s){
	for(auto e: s) if(!isalpha(e)) return false;
	return true;
}

void dbg(){
	rep(i, 1, n){
		rep(j, 1, m) cerr<<f[i][j]<<" ";
		cerr<<endl;
	}
}

bool guass(){
	int rk=0;
	for(int r=1, c=1; r<=n && c<=m; c++){
		int t=r;
		rep(i, r, n) if(f[i][c]){
			t=i;
			break;
		}
		if(!f[t][c]) continue;
		swap(f[t], f[r]);
		col[r]=c;
		++rk;
		
		rep(i, r+1, n) if(f[i][c]){
			int rt=dv(f[i][c], f[r][c]);
			rep(j, c, m) f[i][j]=add(f[i][j], -mul(rt, f[r][j]));
		}
		r++;
	}
	
	dwn(r, rk-1, 1){
		rep(i, r+1, rk){
			int c=col[i];
			int rt=dv(f[r][c], f[i][c]);
			rep(j, c, m) f[r][j]=add(f[r][j], -mul(rt, f[i][j]));
		}
	}
	
	rep(i, 1, n) if(col[i]){
		bool fl=true;
		rep(j, col[i]+1, m) if(f[i][j]) fl=false;
		if(fl) return true;
	}
	return false;
}

signed main(){
	cin>>n;
	getchar();
	
	rep(i, 1, n){
		string s;
		getline(cin, s);
		stringstream ssin(s);
		
		vector<string> buf;
		while(ssin>>s) buf.pb(s);
		
		int sgn=1;
		rep(j, 0, (int)buf.size()-1){
			string cur=buf[j];
			if(cur=="="){
				sgn=-1;
				continue;
			}
			if(par(cur)){
				int fl=1;
				if(j && buf[j-1]=="/") fl=-1;
				int sg=sgn*fl;
				
				int c=get(cur);
				f[i][c]+=sg;
			}
		}
	}
	
	// dbg();
	puts(guass()? "invalid": "valid");
	
	return 0;
}