PAT 1108 Finding Average

#include<bits/stdc++.h>

using namespace std;
int main() {
    int n;
    cin >> n;
    int n_valid = 0;
    double sum = 0;
    for(int i = 0; i < n; ++i) {
        string tmp;
        cin >> tmp;
        bool flag = true;
        try {
            int start = -1;
            int cnt = 0;
            int len = tmp.length();
            int pos;
            while((pos=tmp.find('.', start + 1)) != tmp.npos) {
                ++cnt;
                start = pos;
            }
            if(cnt > 1)
                throw exception();
            if(cnt && len - start - 1 > 2) {
                throw exception();
            }
            double d = stod(tmp);
            if(d > 1000 || d < -1000) {
                throw exception();
            }
            
            sum += d;
            ++n_valid;
        } catch (exception & e) {
            flag = false;
        }
        if(! flag) {
            printf("ERROR: %s is not a legal number\n", tmp.c_str());
        }
    }
    if(n_valid == 0)
        printf("The average of 0 numbers is Undefined\n");
    else if(n_valid > 1)
        printf("The average of %d numbers is %.2f\n", n_valid, sum / n_valid);
    else {
        printf("The average of 1 number is %.2f\n", sum);
    }
}