Uva 442 Matrix Chain Multiplication

#include <algorithm>
#include <iostream>
#include <cstring>  
#include <cstdio>  
#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;
    scanf("%d",&n);
    char s[1010];
    int a,b;
    for(int i=0; i<n; i++){
        scanf("%s %d %d",s, &a, &b);
        m[s[0] - 'A'].a = a;
        m[s[0] - 'A'].b = b;
    }   
    while(scanf("%s",s) != EOF){
        int l = strlen(s), ans = 0;

        bool error = false;
        for(int i=0; i<l; i++){
            if(s[i] >= 'A' && s[i] <= 'Z')
                S.push(m[s[i] - 'A']);
            else if(s[i] == ')'){
                Matrix m2 = S.top(); S.pop();
                Matrix m1 = S.top(); S.pop();
                if(m1.b != m2.a){
                    error = true;
                    break;
                } 
                ans += m1.a * m1.b * m2.b;
                S.push(Matrix(m1.a, m2.b));
            }
        }
        if(error) puts("error");
        else printf("%d\n",ans);
    }
    return 0;
}