为啥会CE？

#include <iostream>
#include<cstring>
using namespace std;
int ma(){
    string s1,s2,a="",b="",ans="";
    int c[5005];
    cin>>s1>>s2;
    for (int i=s1.length()-1;i>=0;i--) a+=s1[i];
    for (int i=s2.length()-1;i>=0;i--) b+=s2[i];
    for (int i=0;i<a.length();i++){
    	for (int j=0;j<b.length();j++){
    		c[i+j]+=(a[i]-'0')*(b[j]-'0');
	    }
    }
    int len=a.length()+b.length()-1;
    for (int i=0;i<=len;i++){
    	c[i+1]+=c[i]/10;
		c[i]%=10;
    }
    while (c[len]==0 && len>0) len--;
    for (int i=len;i>=0;i--) ans+=c[i]+'0';
    int ans1=ans;
    return ans1;
    int an;
int main (){
	string n,s=0;
	cin>>n;
	for(int i=1;i<=n;i++){
		long long k=1;
		for (int j=1;j<=i;j++){
			k*=j;
		}
		   s+=k; 
	}
	cout <<s<<endl;
	return 0;
}