#include<bits/stdc++.h>
#define FOR(i,s,e) for(int i=(s); i<=(e); i++)
#define FORD(i,s,e) for(int i=(s); i>=(e); i--)
#define ALL(k) (k).begin(),(k).end()
#define e1 first
#define e2 second
#define pb push_back


using namespace std;
using LL = long long;
const int MAXN = 1000111;

int edg[5][5];
char a[MAXN],b[MAXN];

int getval(char aa){
	if (aa == 'A') return 1;
	if (aa == 'C') return 2;
	if (aa == 'G') return 3;
	return 4;
}



main(){
	scanf("%s",a+1);
	scanf("%s",b+1);
	int l = strlen(a+1);
	FOR(i,1,l){
		int x = getval(a[i]), y = getval(b[i]);
		edg[x][y]++;
	}
	FOR(x,1,4) edg[x][x] = 0;
	int ans = 0;
	
	FOR(x,1,4){
		FOR(y,1,4){
			if(x==y) continue;
			int ch = min(edg[x][y], edg[y][x]);
			ans += ch;
			edg[x][y] -= ch;
			edg[y][x] -= ch;
		}
	}
	FOR(x,1,4){
		FOR(y,1,4){
			if(x == y) continue;
			FOR(z,1,4){
				if (x == y || y == z || z == x) continue;
				int ch = min(edg[x][y], min(edg[y][z], edg[z][x]));
				ans += 2*ch;
				edg[x][y] -= ch;
				edg[y][z] -= ch;
				edg[z][x] -= ch;
			}
		}
	}
	FOR(x,1,4){
		FOR(y,1,4){
			if(x == y) continue;
			FOR(z,1,4){
				if (x == y || y == z || z == x) continue;
				FOR(t,1,4){
					if(t == x || t == y || t == z) continue;
					int ch = min(min(edg[x][y],edg[y][z]), min(edg[z][t], edg[t][x]));
					ans += 3*ch;
					edg[x][y] -= ch;
					edg[y][z] -= ch;
					edg[z][t] -= ch;
					edg[t][x] -= ch;
				}
			}
		}
	}
	printf("%d\n", ans);
	return 0;
}