string to int

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main()
{
    string s1,s2,s3,s1a="",s2a="",s3a="";
    cin >> s1>>s2>>s3;
    for (int i=0;i<s1.length();i++)
        if (s1[i]>='0' && s1[i]<='9')
           s1a = s1a + s1[i];
     for (int i=0;i<s2.length();i++)
        if (s2[i]>='0' && s2[i]<='9')
           s2a = s2a + s2[i];
     for (int i=0;i<s3.length();i++)
        if (s3[i]>='0' && s3[i]<='9')
           s3a = s3a + s3[i];
    int n1,n2,n3;
    istringstream(s1a) >>n1;
    istringstream(s2a) >> n2;
    istringstream(s3a)  >>n3;
    cout << n1+n2+n3<< endl;

    return 0;
}