Let dp*[j] be the number of number of two letter strings which satisfy the given constraints which are of length i and last character is j. if(j==0) the last character is Y , if (j==1) the last character is X;

Now base case is dp[1][0]=1 and dp[1][1]=0;

//calculating for length>1

```
for(int i=2;i<=N;i++){
dp*[0]=dp*[1]=0;// initializing to 0;
dp*[0]=(dp[i-1][1]+dp[i-1][0])%mod;
//for 'Y' the previous can be anything so adding those values
for(int j=1;j<P;j++){
if(i-j<=0)break;
dp*[1]+=(dp[i-j][0]);
if(dp*[1]>=mod)dp*[1]-=mod;
}
}
```

return (dp[N][0]+dp[N][1])%mod;