AC

#include <iostream>
#include <vector>
#include <string>

using namespace std;

int numDistinct(string A, string B) {

int m = B.size();
int n = A.size();
vector<vector<unsigned long long>> dp(m + 1, vector<unsigned long long>(n + 1, 0));

for (int j = 0; j <= n; ++j) {
    dp[0][j] = 1;
}

for (int i = 1; i <= m; ++i) {
    for (int j = 1; j <= n; ++j) {
        if (B[i - 1] == A[j - 1]) {
            dp[i][j] = dp[i - 1][j - 1] + dp[i][j - 1];
        } else {
            dp[i][j] = dp[i][j - 1];
        }
    }
}

return dp[m][n];

}

int main() {

string A, B;
cin >> A >> B;
cout << numDistinct(A, B) << endl;
return 0;

}