uva 442 Matrix Chain Multiplication

#include<iostream>
#include<cstdio>
#include<cstring>
#include<stack>
using namespace std;
struct Matrix{
int a,b;
Matrix(int a=0,int b=0):a(a),b(b) {};
}m[26];
stack<Matrix> s;
int main()
{
int n;
cin>>n;
for(int i=0;i<n;i++)
{
string name;
cin>>name;
int k=name[0]-'A';
cin>>m[k].a>>m[k].b;
}
string expr;
while(cin>>expr)
{
int len=(int)expr.length();
bool error=false;
int ans=0;
for(int i=0;i<len;i++)
{
if(isalpha(expr[i])) s.push(m[expr[i]-'A']);
else if(expr[i]==')'){
Matrix m1=s.top();s.pop();
Matrix m2=s.top();s.pop();
if(m2.b!=m1.a) {error=true;break;}
ans+=m2.a*m2.b*m1.b;
s.push(Matrix(m2.a,m1.b));
}
}
if(error) cout<<"error"<<endl;
else cout<<ans<<endl;
}
return 0;
}