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munmap_chunk(): invalid pointer

#include<iostream>
#include<string>
using namespace std;
int main(){
    string a,b,a2,b2;
    getline(cin,a);getline(cin,b);
    int lena=a.size(),lenb=b.size(),sum1=0,sum2=0,i;
    for(i=0;i<lena;i++){
        if(islower(a[i]))
            a2[i]=a[i];
        else if(isupper(a[i])){
            a[i]=a[i]+26;
            a2[i]=a[i];
		}
    }
    for(i=0;i<lenb;i++){
        if(islower(b[i]))
            b2[i]=b[i];
        else if(isupper(b[i])){
            b[i]=b[i]+26;
            b2[i]=b[i];
		}
    }
    for(i=0;i<lena;i++){
        if(a[i]!=' ')
            sum1=int(a[i]);
    }
    for(i=0;i<lenb;i++){
        if(b[i]!=' ')
            sum2+=int(b[i]);
    }
    if(sum1==sum2)
        cout << "Yes";
    else
        cout << "NO";
    return 0;
}