func_code_string: #include <bits/stdc++.h>   using namespace std;   long long dp[3505][305];   int a[305], b[305];   const int mod = 1e9 + 7;   int main() {     int n, l;     cin >> n >> l;     for (int i = 0; i < n; i++) {       scanf( %d%d , &a[i], &b[i]);       dp[a[i]][i]++;       if (a[i] != b[i]) {         a[i + n] = b[i];         b[i + n] = a[i];         dp[a[i + n]][i + n]++;       }     }     for (int i = 1; i <= l; i++) {       for (int j = 0; j < n * 2; j++) {         if (dp[i][j] == 0) continue;         for (int k = 0; k < 2 * n; k++) {           if (k % n != j % n && a[k] == b[j]) {             dp[i + a[k]][k] = (dp[i + a[k]][k] + dp[i][j]) % mod;           }         }       }     }     long long ans = 0;     for (int i = 0; i < n * 2; i++) {       ans = (ans + dp[l][i]) % mod;     }     cout << ans << endl;   }   