C++ convert string to int

Used stoi() method to convert string to int, but it converts to int even when there is alpahabet.

string str1 = "45";
string str2 = "31337 test"; 

int myint1 = stoi(str1); // 45
int myint2 = stoi(str2); // 31337

str2 is converted to int, but I don't want this convertion since it has alphabet. If there any way to catch or prevent this conversion.