Codeforces 163A Substring and Subsequence

#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
#include<iostream>
const int Mod=1000000007;
char a[500005],b[500005];
int n,m,f[5005][5005];
int read(){
int t=0,f=1;char ch=getchar();
while (ch<'0'||ch>'9'){if (ch=='-') f=-1;ch=getchar();}
while ('0'<=ch&&ch<='9'){t=t*10+ch-'0';ch=getchar();}
return t*f;
}
int main(){
scanf("%s",a+1);n=strlen(a+1);
scanf("%s",b+1);m=strlen(b+1);
for (int i=1;i<=n;i++)
if (a[i]==b[1]) f[i][1]=1;
for (int i=1;i<=m;i++)
if (a[1]==b[i]) f[1][i]=1;
int ans=0;
for (int i=1;i<=n;i++){
for (int j=1;j<=m;j++){
f[i][j]=f[i][j-1];
if (a[i]==b[j])
(f[i][j]+=f[i-1][j-1]+1)%=Mod;
}
ans=(ans+f[i][m])%Mod;
}
printf("%d\n",ans);
}