为什么20

#include<iostream>
#include<cstring>
#include<cstdio>
using namespace std;

int main(){
    char a1[100000], b1[100000];
    int a[100000], b[100000], c[100000], lena, lenb, lenc, i, j, x;
    memset(a, 0, sizeof(a));
    memset(b, 0, sizeof(b));
    memset(c, 0, sizeof(c));
    scanf("%s%s", &a1, &b1);
    lena = strlen(a1);
    lenb = strlen(b1);
    for(i = 0; i < lena; i++)
      a[lena - i] = a1[i] - 48;
    for(i = 0; i < lenb; i++)
      b[lenb - i] = b1[i] - 48;
    for(i = 1; i <= lena; i++){
        x = 0;
        for(j = 1; j <= lenb; j++){
            c[i + j -1] = a[i] * b[i] + x + c[i + j - 1];
            x = c[i + j - 1] / 10;
            c[i + j -1] %= 10; 
        }
        c[i + lenb] = x;
    }
    lenc = lena + lenb;
    while(c[lenc] == 0 && lenc > 1)
      lenc--;
    for(i = lenc; i >= 1; i--)
      cout << c[i];
    cout << endl;
    return 0;
}