求教大佬，我的程序为什么会出错，怎么该啊？

#include <iostream>
#include <queue>
#include <cstring>
#include <cmath>
using namespace std;
void mul(int a[],int b[]){
    int ans[1001];
    memset(ans,0, sizeof(ans));
    for(register int i = 1;i<=500;i++)
        for(register int j = 1;j<=500;j++)
            ans[i+j-1] += a[i]*b[j];
    for(int i = 1;i<=500;i++){
        ans[i+1]+=ans[i]/10;
        ans[i]%=10;
    }
    for(int i = 1;i<=500;i++)a[i] = ans[i];
}
int main() {
    int p;
    cin>>p;
    cout<<ceil(log10(2)*p)<<endl;
    int a[501];
    int c[501];
    memset(a,0,sizeof(a));
    memset(c,0,sizeof(c));
    a[1] = c[1] = 2;
    while(p>=1){
        if(p==1){
            mul(a,c);
            break;
        }
        if(p%2==1)mul(a,c);
        p=p>>1;
        mul(a,a);
    }

    for(int i = 500;i>=1;i--)cout<<a[i];
    return 0;
}